Make GlobalCss usable outside of <App>
User-Facing Changes
Description
A different approach to https://github.com/foxglove/studio/pull/3788 - the main takeaway is that we still have some Fluent? Or other context menus that need the body to have the correct fonts because that's where they put their overlays and don't wrap them with a theme provider.
Until we resolve that, we can't remove the fonts from the body. Once we can remove the fonts from the body, then we can remove the dependency on theme and allow GlobalCss to be opt-in outside of App.

@2metres is my analysis on why we need the fonts in the body correct here? It is a fluent thing and that for mui we are ok cause mui puts its context menus under a theme provider?
@defunctzombie I believe that is correct. MUI puts it's poppers just inside the root element whereas Fluent UI puts it in the body
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This PR's time will come. It waits as a reminder to me.