[WIP] Dark mode

[Mnaukal]

Work in progress, do not merge yet!

Dark mode implementation with data-bs-theme attribute from Bootstrap 5.3. Some custom style modifications are also necessary.

The color theme (light/dark) is applied to the Layout element. For now, we use the userUIData.darkTheme option (originally meant only for source code editor theme) to save the color theme preference to the user account.

TODO

• Should we create separate user settings for app dark theme and source code editor dark theme?

Styles that need to be fixed

• Modals (they are outside the Layout element, so the data-bs-theme is not applied to them)
• Dropdowns
• Code compare page
• hover in tables (td.clickable:hover, ... in recodex.css)