Dark mode

[wooster0]

Is your feature request related to a problem? Please describe.
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The problem is that the default theme is a little bit too white in my opinion. Especially late at night it can be pretty dazzling I think.

Describe the solution you'd like
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I would like there to be a dark mode/theme.

Describe alternatives you've considered
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Making the default theme/appearance darker.

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I switched main-color and main-bg in the CSS and it already looked a lot better.