recent_topics: Change background color in dark mode on hover on a row.

[shubham-padia]

Fixes #30083.
We have used the dark theme color for the left sidebar row hover: --color-background-active-narrow-filter. We are not using that variable directly since we don't want a change in that color affecting the color in recent conversations.
We've also used a single color on hover compared to 2 different colors for unread and read row before. Light mode also uses a single color on hover.

Relevant CZO convo: link

Screenshots and screen captures:

Hover GIF

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[timabbott]

Hmm, I think it might be better to just use the CSS variable; if we change the color, we'll grep for uses of the variable and ask the question of what our intent is; whereas if we

Also small comment: Let's always cross-link the chat.zulip.org thread for this sort of thing.

[shubham-padia]

Hmm, I think it might be better to just use the CSS variable; if we change the color, we'll grep for uses of the variable and ask the question of what our intent is; whereas if we

I think the sentence is incomplete here 😅

[timabbott]

Yeah I meant to say "whereas if we just use the same value, we'll probably not ask the question"

[timabbott]

Merged, thanks @shubham-padia!