diff --git a/src/pydata_sphinx_theme/assets/styles/variables/_color.scss b/src/pydata_sphinx_theme/assets/styles/variables/_color.scss
index c7b85341c..08ddb4d43 100644
--- a/src/pydata_sphinx_theme/assets/styles/variables/_color.scss
+++ b/src/pydata_sphinx_theme/assets/styles/variables/_color.scss
@@ -80,53 +80,71 @@ $pst-semantic-colors: (
 
 /*******************************************************************************
 * write the color rules for each theme (light/dark)
-*
-* NOTE: @each {...} is like a for-loop
-* https://sass-lang.com/documentation/at-rules/control/each
-* and #{...} inserts a variable into a CSS selector or property name
-* https://sass-lang.com/documentation/interpolation
 */
-@each $mode in (light, dark) {
-  html[data-theme="#{$mode}"] {
-    @each $name, $value in $pst-semantic-colors {
-      // check if this color is defined differently for light/dark
-      @if type-of($value) == map {
-        $value: map-get($value, $mode);
-      }
+
+/* NOTE:
+ * Mixins enable us to reuse the same definitions for the different modes
+ * https://sass-lang.com/documentation/at-rules/mixin
+ * #{...} inserts a variable into a CSS selector or property name
+ * https://sass-lang.com/documentation/interpolation
+ */
+@mixin theme-colors($mode) {
+  // check if this color is defined differently for light/dark
+  @each $name, $value in $pst-semantic-colors {
+    @if type-of($value) == map {
+      $value: map-get($value, $mode);
+    }
+    & {
       --pst-color-#{$name}: #{$value};
     }
-    // assign the "duplicate" colors (ones that just reference other variables)
+  }
+  // assign the "duplicate" colors (ones that just reference other variables)
+  & {
     --pst-color-link: var(--pst-color-primary);
     --pst-color-link-hover: var(--pst-color-warning);
-    // adapt to light/dark-specific content
-    @if $mode == "light" {
-      .only-dark {
-        display: none !important;
-      }
-    } @else {
-      .only-light {
-        display: none !important;
-      }
-      /* Adjust images in dark mode (unless they have class .only-dark or
-       * .dark-light, in which case assume they're already optimized for dark
-       * mode).
-       */
-      img:not(.only-dark):not(.dark-light) {
-        filter: brightness(0.8) contrast(1.2);
-      }
-      /* Give images a light background in dark mode in case they have
-      *  transparency and black text (unless they have class .only-dark or .dark-light, in
-      *  which case assume they're already optimized for dark mode).
-      */
-      .bd-content img:not(.only-dark):not(.dark-light) {
-        background: rgb(255, 255, 255);
-        border-radius: 0.25rem;
-      }
-      // MathJax SVG outputs should be filled to same color as text.
-      .MathJax_SVG * {
-        fill: var(--pst-color-text-base);
-      }
+  }
+  // adapt to light/dark-specific content
+  @if $mode == "light" {
+    .only-dark {
+      display: none !important;
+    }
+  } @else {
+    .only-light {
+      display: none !important;
+    }
+    /* Adjust images in dark mode (unless they have class .only-dark or
+     * .dark-light, in which case assume they're already optimized for dark
+     * mode).
+     */
+    img:not(.only-dark):not(.dark-light) {
+      filter: brightness(0.8) contrast(1.2);
     }
+    /* Give images a light background in dark mode in case they have
+    *  transparency and black text (unless they have class .only-dark or .dark-light, in
+    *  which case assume they're already optimized for dark mode).
+    */
+    .bd-content img:not(.only-dark):not(.dark-light) {
+      background: rgb(255, 255, 255);
+      border-radius: 0.25rem;
+    }
+    // MathJax SVG outputs should be filled to same color as text.
+    .MathJax_SVG * {
+      fill: var(--pst-color-text-base);
+    }
+  }
+}
+
+/* Defaults to light mode if data-theme is not set */
+html:not([data-theme]) {
+  @include theme-colors("light");
+}
+
+/* NOTE: @each {...} is like a for-loop
+ * https://sass-lang.com/documentation/at-rules/control/each
+ */
+@each $mode in (light, dark) {
+  html[data-theme="#{$mode}"] {
+    @include theme-colors($mode);
   }
 }
 
diff --git a/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/navbar-logo.html b/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/navbar-logo.html
index c96399788..6336e1371 100644
--- a/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/navbar-logo.html
+++ b/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/navbar-logo.html
@@ -13,8 +13,16 @@
   {% set is_logo = "light" in theme_logo["image_relative"] %}
   {% set alt = theme_logo.get("alt_text", "Logo image") %}
   {% if is_logo %}
-    <img src="{{ theme_logo['image_relative']['light'] }}" class="logo__image only-light" alt="{{ alt }}"/>
-    <img src="{{ theme_logo['image_relative']['dark'] }}" class="logo__image only-dark" alt="{{ alt }}"/>
+    {# Theme switching is only available when JavaScript is enabled.
+     # Thus we should add the extra image using JavaScript, defaulting
+     # depending on the value of default_mode; and light if unset.
+     #}
+    {% if default_mode is undefined %}
+      {% set default_mode = "light" %}
+    {% endif %}
+    {% set js_mode = "light" if default_mode == "dark" else "dark" %}
+    <img src="{{ theme_logo['image_relative'][default_mode] }}" class="logo__image only-{{ default_mode }}" alt="{{ alt }}"/>
+    <script>document.write(`<img src="{{ theme_logo['image_relative'][js_mode] }}" class="logo__image only-{{ js_mode }}" alt="{{ alt }}"/>`);</script>
   {% endif %}
   {% if not is_logo or theme_logo.get("text") %}
     <p class="title logo__title">{{ theme_logo.get("text") or docstitle }}</p>
diff --git a/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/search-button.html b/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/search-button.html
index 13262852b..76fc231f9 100644
--- a/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/search-button.html
+++ b/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/search-button.html
@@ -1,4 +1,11 @@
-{# A button that, when clicked, will trigger a search popup overlay #}
-<button class="btn btn-sm navbar-btn search-button search-button__button" title="{{ _('Search') }}" aria-label="{{ _('Search') }}" data-bs-placement="bottom" data-bs-toggle="tooltip">
-  <i class="fa-solid fa-magnifying-glass"></i>
-</button>
+{# A button that, when clicked, will trigger a search popup overlay.
+ #
+ # As this function will only work when JavaScript is enabled, we add it through JavaScript.
+ #}
+<script>
+document.write(`
+  <button class="btn btn-sm navbar-btn search-button search-button__button" title="{{ _('Search') }}" aria-label="{{ _('Search') }}" data-bs-placement="bottom" data-bs-toggle="tooltip">
+    <i class="fa-solid fa-magnifying-glass"></i>
+  </button>
+`);
+</script>
diff --git a/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/theme-switcher.html b/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/theme-switcher.html
index 65c483166..63c8cd239 100644
--- a/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/theme-switcher.html
+++ b/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/theme-switcher.html
@@ -1,5 +1,11 @@
-<button class="theme-switch-button btn btn-sm btn-outline-primary navbar-btn rounded-circle" title="{{ _('light/dark') }}" aria-label="{{ _('light/dark') }}" data-bs-placement="bottom" data-bs-toggle="tooltip">
+{# As the theme switcher will only work when JavaScript is enabled, we add it through JavaScript.
+ #}
+<script>
+document.write(`
+  <button class="theme-switch-button btn btn-sm btn-outline-primary navbar-btn rounded-circle" title="{{ _('light/dark') }}" aria-label="{{ _('light/dark') }}" data-bs-placement="bottom" data-bs-toggle="tooltip">
     <span class="theme-switch" data-mode="light"><i class="fa-solid fa-sun"></i></span>
     <span class="theme-switch" data-mode="dark"><i class="fa-solid fa-moon"></i></span>
     <span class="theme-switch" data-mode="auto"><i class="fa-solid fa-circle-half-stroke"></i></span>
-</button>
+  </button>
+`);
+</script>
diff --git a/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/version-switcher.html b/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/version-switcher.html
index 6a7f43871..0085d070a 100644
--- a/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/version-switcher.html
+++ b/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/components/version-switcher.html
@@ -1,9 +1,15 @@
-<div class="version-switcher__container dropdown">
+{# As the version switcher will only work when JavaScript is enabled, we add it through JavaScript.
+ #}
+<script>
+document.write(`
+  <div class="version-switcher__container dropdown">
     <button type="button" class="version-switcher__button btn btn-sm navbar-btn dropdown-toggle" data-bs-toggle="dropdown">
-        {{ theme_switcher.get('version_match') }}  <!-- this text may get changed later by javascript -->
-        <span class="caret"></span>
+      {{ theme_switcher.get('version_match') }}  <!-- this text may get changed later by javascript -->
+      <span class="caret"></span>
     </button>
     <div class="version-switcher__menu dropdown-menu list-group-flush py-0">
     <!-- dropdown will be populated by javascript on page load -->
+    </div>
   </div>
-</div>
+`);
+</script>
diff --git a/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/layout.html b/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/layout.html
index 5c10712c7..ec88af879 100644
--- a/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/layout.html
+++ b/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/layout.html
@@ -1,3 +1,9 @@
+{# We redefine <html/> for "basic/layout.html" to add a default `data-theme` attribute when
+ # a default mode has been set. This also improves compatibility when JavaScript is disabled.
+ #}
+{% set html_tag %}
+<html{% if not html5_doctype %} xmlns="http://www.w3.org/1999/xhtml"{% endif %}{% if language is not none %} lang="{{ language }}"{% endif %} {% if default_mode %}data-theme="{{ default_mode }}"{% endif %}>
+{% endset %}
 {%- extends "basic/layout.html" %}
 {%- import "static/webpack-macros.html" as _webpack with context %}
 {# Metadata and asset linking #}
@@ -64,10 +70,7 @@
     <div class="search-button__search-container">{% include "../components/search-field.html" %}</div>
   </div>
   {%- if theme_announcement -%}
-    <div class="bd-header-announcement container-fluid"
-          id="header-announcement">
-      {% include "sections/announcement.html" %}
-    </div>
+    {% include "sections/announcement.html" %}
   {%- endif %}
   {% block docs_navbar %}
     <nav class="bd-header navbar navbar-expand-lg bd-navbar" id="navbar-main">
diff --git a/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/sections/announcement.html b/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/sections/announcement.html
index 2d5c47b86..85d5957e8 100644
--- a/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/sections/announcement.html
+++ b/src/pydata_sphinx_theme/theme/pydata_sphinx_theme/sections/announcement.html
@@ -1,11 +1,14 @@
+{% set header_classes = ["bd-header-announcement", "container-fluid"] %}
 {% set is_remote=theme_announcement.startswith("http") %}
 {# If we are remote, add a script to make an HTTP request for the value on page load #}
 {%- if is_remote %}
 <script>
+document.write(`<div id="header-announcement"></div>`);
 fetch("{{ theme_announcement }}")
   .then(res => {return res.text();})
   .then(data => {
     div = document.querySelector("#header-announcement");
+    div.classList.add(...{{ header_classes | tojson }});
     div.innerHTML = `<div class="bd-header-announcement__content">${data}</div>`;
   })
   .catch(error => {
@@ -14,5 +17,7 @@
 </script>
 {#- if announcement text is not remote, populate announcement w/ local content -#}
 {%- else %}
-  <div class="bd-header-announcement__content">{{ theme_announcement }}</div>
+  <div class="{{ header_classes | join(' ') }}" id="header-announcement">
+    <div class="bd-header-announcement__content">{{ theme_announcement }}</div>
+  </div>
 {% endif %}
diff --git a/tests/test_build.py b/tests/test_build.py
index d1133ac5e..0132cd57c 100644
--- a/tests/test_build.py
+++ b/tests/test_build.py
@@ -201,6 +201,51 @@ def test_logo_two_images(sphinx_build_factory):
     assert "Foo Title" in index_str
 
 
+def test_primary_logo_is_light_when_no_default_mode(sphinx_build_factory):
+    """Test that the primary logo image is light
+    (and secondary, written through JavaScript, is dark)
+    when no default mode is set."""
+    # Ensure no default mode is set
+    confoverrides = {
+        "html_context": {},
+    }
+    sphinx_build = sphinx_build_factory("base", confoverrides=confoverrides).build()
+    index_html = sphinx_build.html_tree("index.html")
+    navbar_brand = index_html.select(".navbar-brand")[0]
+    assert navbar_brand.find("img", class_="only-light") is not None
+    assert navbar_brand.find("script", string=re.compile("only-dark")) is not None
+
+
+def test_primary_logo_is_light_when_default_mode_is_light(sphinx_build_factory):
+    """Test that the primary logo image is light
+    (and secondary, written through JavaScript, is dark)
+    when default mode is set to light."""
+    # Ensure no default mode is set
+    confoverrides = {
+        "html_context": {"default_mode": "light"},
+    }
+    sphinx_build = sphinx_build_factory("base", confoverrides=confoverrides).build()
+    index_html = sphinx_build.html_tree("index.html")
+    navbar_brand = index_html.select(".navbar-brand")[0]
+    assert navbar_brand.find("img", class_="only-light") is not None
+    assert navbar_brand.find("script", string=re.compile("only-dark")) is not None
+
+
+def test_primary_logo_is_dark_when_default_mode_is_dark(sphinx_build_factory):
+    """Test that the primary logo image is dark
+    (and secondary, written through JavaScript, is light)
+    when default mode is set to dark."""
+    # Ensure no default mode is set
+    confoverrides = {
+        "html_context": {"default_mode": "dark"},
+    }
+    sphinx_build = sphinx_build_factory("base", confoverrides=confoverrides).build()
+    index_html = sphinx_build.html_tree("index.html")
+    navbar_brand = index_html.select(".navbar-brand")[0]
+    assert navbar_brand.find("img", class_="only-dark") is not None
+    assert navbar_brand.find("script", string=re.compile("only-light")) is not None
+
+
 def test_logo_missing_image(sphinx_build_factory):
     """Test that a missing image will raise a warning."""
     # Test with a specified title and a dark logo
@@ -665,7 +710,9 @@ def test_version_switcher(sphinx_build_factory, file_regression, url):
 
     if url == "switcher.json":  # this should work
         index = sphinx_build.html_tree("index.html")
-        switcher = index.select(".version-switcher__container")[0]
+        switcher = index.select(".navbar-header-items")[0].find(
+            "script", string=re.compile(".version-switcher__container")
+        )
         file_regression.check(
             switcher.prettify(), basename="navbar_switcher", extension=".html"
         )
@@ -683,7 +730,11 @@ def test_theme_switcher(sphinx_build_factory, file_regression):
     """Regression test the theme switcher btn HTML"""
 
     sphinx_build = sphinx_build_factory("base").build()
-    switcher = sphinx_build.html_tree("index.html").select(".theme-switch-button")[0]
+    switcher = (
+        sphinx_build.html_tree("index.html")
+        .find(string=re.compile("theme-switch-button"))
+        .find_parent("script")
+    )
     file_regression.check(
         switcher.prettify(), basename="navbar_theme", extension=".html"
     )
diff --git a/tests/test_build/navbar_switcher.html b/tests/test_build/navbar_switcher.html
index 02a2965bd..ae125a111 100644
--- a/tests/test_build/navbar_switcher.html
+++ b/tests/test_build/navbar_switcher.html
@@ -1,11 +1,13 @@
-<div class="version-switcher__container dropdown">
- <button class="version-switcher__button btn btn-sm navbar-btn dropdown-toggle" data-bs-toggle="dropdown" type="button">
-  0.7.1
-  <!-- this text may get changed later by javascript -->
-  <span class="caret">
-  </span>
- </button>
- <div class="version-switcher__menu dropdown-menu list-group-flush py-0">
-  <!-- dropdown will be populated by javascript on page load -->
- </div>
-</div>
+<script>
+ document.write(`
+  <div class="version-switcher__container dropdown">
+    <button type="button" class="version-switcher__button btn btn-sm navbar-btn dropdown-toggle" data-bs-toggle="dropdown">
+      0.7.1  <!-- this text may get changed later by javascript -->
+      <span class="caret"></span>
+    </button>
+    <div class="version-switcher__menu dropdown-menu list-group-flush py-0">
+    <!-- dropdown will be populated by javascript on page load -->
+    </div>
+  </div>
+`);
+</script>
diff --git a/tests/test_build/navbar_theme.html b/tests/test_build/navbar_theme.html
index 61345ebd2..e3dfce411 100644
--- a/tests/test_build/navbar_theme.html
+++ b/tests/test_build/navbar_theme.html
@@ -1,14 +1,9 @@
-<button aria-label="light/dark" class="theme-switch-button btn btn-sm btn-outline-primary navbar-btn rounded-circle" data-bs-placement="bottom" data-bs-toggle="tooltip" title="light/dark">
- <span class="theme-switch" data-mode="light">
-  <i class="fa-solid fa-sun">
-  </i>
- </span>
- <span class="theme-switch" data-mode="dark">
-  <i class="fa-solid fa-moon">
-  </i>
- </span>
- <span class="theme-switch" data-mode="auto">
-  <i class="fa-solid fa-circle-half-stroke">
-  </i>
- </span>
-</button>
+<script>
+ document.write(`
+  <button class="theme-switch-button btn btn-sm btn-outline-primary navbar-btn rounded-circle" title="light/dark" aria-label="light/dark" data-bs-placement="bottom" data-bs-toggle="tooltip">
+    <span class="theme-switch" data-mode="light"><i class="fa-solid fa-sun"></i></span>
+    <span class="theme-switch" data-mode="dark"><i class="fa-solid fa-moon"></i></span>
+    <span class="theme-switch" data-mode="auto"><i class="fa-solid fa-circle-half-stroke"></i></span>
+  </button>
+`);
+</script>
diff --git a/tests/test_build/sidebar_subpage.html b/tests/test_build/sidebar_subpage.html
index 89c4b9d92..ff7fd9910 100644
--- a/tests/test_build/sidebar_subpage.html
+++ b/tests/test_build/sidebar_subpage.html
@@ -36,20 +36,15 @@
   </div>
   <div class="sidebar-header-items__end">
    <div class="navbar-end-item">
-    <button aria-label="light/dark" class="theme-switch-button btn btn-sm btn-outline-primary navbar-btn rounded-circle" data-bs-placement="bottom" data-bs-toggle="tooltip" title="light/dark">
-     <span class="theme-switch" data-mode="light">
-      <i class="fa-solid fa-sun">
-      </i>
-     </span>
-     <span class="theme-switch" data-mode="dark">
-      <i class="fa-solid fa-moon">
-      </i>
-     </span>
-     <span class="theme-switch" data-mode="auto">
-      <i class="fa-solid fa-circle-half-stroke">
-      </i>
-     </span>
-    </button>
+    <script>
+     document.write(`
+  <button class="theme-switch-button btn btn-sm btn-outline-primary navbar-btn rounded-circle" title="light/dark" aria-label="light/dark" data-bs-placement="bottom" data-bs-toggle="tooltip">
+    <span class="theme-switch" data-mode="light"><i class="fa-solid fa-sun"></i></span>
+    <span class="theme-switch" data-mode="dark"><i class="fa-solid fa-moon"></i></span>
+    <span class="theme-switch" data-mode="auto"><i class="fa-solid fa-circle-half-stroke"></i></span>
+  </button>
+`);
+    </script>
    </div>
    <div class="navbar-end-item">
     <ul aria-label="Icon Links" class="navbar-nav" id="navbar-icon-links">