Audit color usage in product CSS

Identify custom and page-specific color usage so we can track and figure out how to approach these area with regard to tokens (and eventually dark mode)

A good starting point could be: https://gitlab-org.gitlab.io/frontend/playground/gitlab-code-scanner/?search=check_id%20co%20%22color%22

We want to get a sense of how many colour classes are used. If we can break this up into type, like color, border-color and background-color usage, that would help, but not essential. Within an order of magnitude would be good enough.

@danmh "If not all dark mode colors become design tokens, hopefully issue can determine the engineering approach to handling these non-tokened colour changes."