Rustdoc: button text does not respect color themes

[LeSeulArtichaut]

I visited nightly-generated docs, e.g. https://doc.rust-lang.org/nightly/std/?search=a

With the dark and ayu themes, some texts appear black although they shouldn't. This includes:

• tab buttons in searches ("in names", "in parameters", "in return types")
• theme selection buttons, including those in the settings page

IIRC this wasn't the case yesterday, so this must have regressed in today's nightly (2021-02-26)

[LeSeulArtichaut]

I think this might be due to #82313? cc @jyn514