Add support for dark mode

[MarekPikula]

I would like to use this module with a theme which natively supports dark mode (namely Stack). Unfortunately, as I can see, it doesn't change the colours when the dark mode is enabled.

This is how it looks like with the base theme in dark mode:

It's not terrible, but it would be nice if the colour scheme changed. Do you think it's easy to implement, or should I request it in the theme itself?