Rustdoc question mark next to search not visible in dark theme

[Nashenas88]

The new question mark that was added next to the search box with 1.47 does not change its color when switching to the dark theme. The colors are so similar that the button just appears as a white box unless you look really close.

[ollie27]

This was fixed by #75837 which I guess should have been backported. Either way this is fixed in 1.48.