[v2] iterate over colors and ensure accessible contrasts

We should give the new dark mode (maybe also the light one) another pass to ensure we have accessible colors and good contrasts.

No files, tests, or entry points are named. Start by locating the dark-mode and possibly light-mode color definitions, then check their contrast ratios against the project's accessibility expectations. Done means the relevant theme colors have been reviewed and the accessible-contrast goal is verified.

Written by the indexing model from the issue text.