Dark mode

[olaven]

Is your feature request related to a problem? Please describe.
A lot of people, me included, prefer dark mode. This is not just a preference, but also an
accessibility feature for some.

Describe the solution you'd like
An option to activate style sheets with a button that is added to the page.
This would come in addition to the existing style sheet.
It is added if the user specifies a "dark_mode"-style sheet in options.

Describe alternatives you've considered

• having a theme (or similar) solve this, but that means you cannot have both light and dark mode at the same time
• having a different style sheet that replaces the old one, but that would mean repeating a lot of styles

EDIT: fixed a lot of typos.