Fix low contrast in 'Hide search matches' box in dark mode

[melissawm]

I think this will fix the issue, but the search does not work locally so I will do a mock deployment to my fork and link here when that's done.

EDIT: Looks like it works, can you check @dstansby? https://melissawm.github.io/napari.github.io/dev/developers/contributing/testing.html

[dstansby]

Looks good to me! 🚀