Rollup merge of #79973 - camelid:rustdoc-search-tab-color, r=GuillaumeGomez
Rollup merge of #79973 - camelid:rustdoc-search-tab-color, r=GuillaumeGomez

rustdoc light theme: Fix CSS for selected buttons

Fixes #79961.

The background was dark before, which made the text impossible to read.
Now the button doesn't override the background, and the only thing it
does is add a light-blue top border.

Ultimately, the search results tabs now look very similar to how they
used to look.

r? `@GuillaumeGomez`

graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust