(follow-up from #47810) rustdoc: minor style nits in mobile mode

[QuietMisdreavus]

(stolen from #47810 (comment) now that the PR is merged)

The padding on the top bar seems to be rendered in a different color? It's really subtle in the dark theme, but in the light theme it sticks out. Strangely, when viewing this in Chrome, the color resets to the "proper" one when i start mousing over elements in the inspector (which causes them to be highlighted in the page), so this may be a Chrome thing.

I like how the theme button is next to the search box now, but i guess i expected it to disappear under the menu when it's open? Right now it's just hovering awkwardly over the search box.

cc @GuillaumeGomez

[GuillaumeGomez]

Thanks for the issue report! I'll fix it in a few days.