Search: Input text is white on white on dark mode

[mhilbrunner]

Search: Input text is white on white on dark mode OS/browsers. It also seems the --readthedocs-search-color CSS var isn't actually set for us on https://docs.godotengine.org/en/latest, so this may be an issue on our end?

For now, I've disabled the search addon due to #419.

[rjwills28]

Also having this problem that the search input text is white and not visible in dark mode.