Dark theme for web pages

[VMelnalksnis]

Describe the solution you'd like
Gnomeshade web pages have dark theme support, and theme is automatically selected based on browser settings.

Additional context
The documentation pages already have a CSS implementation for light/dark theme, but Gnomeshade uses bootstrap 5, so most likely will need a different implementation. Also need to consider swagger UI, the maintainers have said that they won't support dark theme, so will also have to look for a custom solution.