Update switcher styling

[drammock]

addresses some of the issues in the comments of #1387, related to the version switcher:

• "stable" is no longer always green
• fix mismatch between button and dropdown menu (one said "latest" other said "dev")
• add left border (like on admonitions) to the active dropdown item, so it's status as "active" does not rely only on colour
• changes the colour of the active item from info to primary

[drammock]

here's a screenshot from a local build (since PR-builds always fail version-match, no item will be "active") --- note that my cursor is on the "stable" item, but isn't visible in the screenshot:

[trallard]

Yay! Thanks for fixing this @drammock - I left a note about using the mixin instead for consistency, and we should be able to merge.

[trallard]

Thank you, looks good to me so will merge 🚀