Add a dark theme

[jtojnar]

Selfoss is too light for us night owls. It would be nice to add a dark theme.

The preference should be auto-detected. And there should be an option to force either light or dark theme.

The main question is how to deal with images. They often assume they are on a light background and contain either e.g. black text on a transparent background, making them illegible in dark theme, or they have a opaque white background, making them blinding.

Previous attempts:

• https://github.com/fossar/selfoss/issues/1094#issuecomment-466178382
• https://github.com/fossar/selfoss/issues/1303

Also related:

• color-scheme