x/website/tour: dark mode file name colors change on hover / click

[suzmue]

Navigate to https://go.dev/tour/welcome/1 and switch to dark mode. Hover and click on the hello.go filename.

Normal:

Hover:

Click:

In light mode, hovering over the file name does not have a visual effect.

[gopherbot]

Change https://go.dev/cl/735400 mentions this issue: _content/tour/static/css/app: make dark mode more pleasant and readable