Adapt appearance of embedded web views to dark mode

[mathebox]

Is your feature request related to a problem? Please describe.
When switching to dark mode, the embedded web view still show light content. This breaks the overall UI.

Describe the solution you'd like
Let's try to override the colors of the shown webpage by injecting some javascript code which overrides the colors.

Enable dark color scheme:

:root {
    color-scheme: light dark;
}

Use media query for automatic switching(?):

@media (prefers-color-scheme: dark) {
    // ...
}

Describe alternatives you've considered
Support dark mode platform-wide. But this would require more development resources (https://dev.xikolo.de/youtrack/issue/XI-3961)

Affected Component
WebViewController

Additional context
This does only apply to the course areas pinboard and recap for now.

Some useful links:
https://developer.apple.com/videos/play/wwdc2019/511
https://stackoverflow.com/questions/33123093/insert-css-into-loaded-html-in-uiwebview-wkwebview#33126467