Themes documentation page is outdated with missing themes and broken screenshots

[ashmod]

The themes documentation page is outdated in a few ways:

Missing themes

The following built-in themes are not listed on the page:

• Holiday (dark)
• Shades Of Purple (dark)

Broken/missing image assets

• theme-solarized-dark.png and theme-solarized-light.png were referenced in the docs but did not exist in docs/assets/, resulting in broken images on the live site.

Outdated screenshots

• The existing dark theme screenshots used an older version of the UI (e.g. different theme selector highlight text color).

[gemini-cli]

👋 @ashmod, you've been assigned to this issue! Thank you for taking the time to contribute. Make sure to check out our contributing guidelines.

[cocosheng-g]

review in progress