rustdoc: theme switcher button in awkward locations on mobile

[QuietMisdreavus]

When viewing docs on mobile, the "switch theme" button is placed into the top bar. However, this overlaps the crate version indicator on the crate root:

When the sidebar menu is opened, the button is instead placed into that sidebar menu, but can also overlap text there:

[GuillaumeGomez]

That's a funny one!