Fix button text color in dark mode

Replace some usages of $white (which is #333 in dark mode) with $white-contrast (which is always #fff).

Also delete unused deprecated-adapter rule

Previously the classes like btn-info were overriding text color on hover/focus.

How to test

1. Enable Dark mode
2. Go to repository, view a file
3. Hover the Edit button
4. Check text color

before	after