Clean up dark_theme.css.

As of this posting (2026-05-20), dark_theme.css is down to about 325 lines. We should strive to clean it up further, until all that remains are dark-theme adjustments that are too complicated to express as CSS variables.

There are at least the following categories of cleanup to be done:

• Identifying any dead or unreachable selectors in dark_theme.css and removing them.
• Expressing all remaining color values (e.g., color: hsl(236deg 33% 90%)) as CSS variables; it's worth searching app_variables.css for any inline color values like that to see if there are existing variables that can be dropped in.
• Squaring existing dark-theme selectors (e.g., .enter_sends_minor) with their light-mode counterparts. In some cases, it's likely that we're using a more specific/targeted selector than in light theme to make a dark-theme adjustment. Ideally, we should have a single authoritative selector in light theme that uses a variable and something like light-dark() to express the dark-theme difference.
• Looking at any dark-theme adjustments that are so minor as to be unnecessary. Any such adjustments should be discussed in #design

In all but the removal of minor adjustments, PRs with any of these fixes should have precise before/after screenshots in both light and dark theme to demonstrate that the cleanup does not introduce any visual changes or regressions.