Support dark theme in editor

The editor in the translation app doesn't support dark mode. For information, other inputs support dark mode so we could reuse the same colors for the editor. Additionally, we might have to tweak the `--tags-*` colors too.

<img width="1759" height="661" alt="Image" src="https://github.com/user-attachments/assets/6f7d6620-1c3f-42c2-9098-631402fcf31a" />