[FR] Enable dark mode

[Mrugalla]

Is your feature request related to a problem? Please describe.
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My eyes hurt when using bright plugins at night. Dark mode solves that.

Describe the solution you'd like
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A button (or other feature) that inverts all colours and saves this state, so that new plugin instances also generally get instantiated with dark mode.

Describe alternatives you've considered
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Making the plugin interface smaller.