Docs: Make theme switcher accessible (#37780)

Docs: Make theme switcher accessible (#37780)

* Make theme switcher accessible

* set an explicit `aria-label` to the switcher (as the `<span>` is not sufficient, as it can be display:none'd and then the button has no accName)
* make the theme buttons actual `aria-pressed` toggles

* Dynamically update aria-label for theme switcher

* Explicitly reset focus after activating theme

* Use innerText for the constructed dynamic aria-label

this way, if the text ever gets changed in the html, this will adapt appropriately

* Tweak accessible name for the dropdown

* Fixup

* Use `textContent` instead of `innerText`