feat: add light/dark mode switch (#1874)

[nerd4me]

add light/dark mode switch (#1874)

[yagebu]

Hi, thanks for creating this PR. There's some problems I see with this current approach:

• I think the screen space in the header is too valuable for this sort of infrequently used toggle. I think putting it on the options page would be better.
• The default shouldn't be dark but whatever the system default is. So there would need to be three toggle options. AFAICT there's no good way to implement this without having to duplicate all the styles... Generally, I guess it's annoying that this is not simply a browser feature...

One other point: Did you create those images yourself?