Support for dark bootswatch themes

[luyangkenneth]

A follow-up from #745

#547 contains details on enabling dark theme support for custom components

[ang-zeyu]

This and #903 are mutually exclusive.

Closing in favour of #903, as bootswatch themes have been so far rather unmaintainable (too many added position/borders/margins/... what not's to take into account other than just colors)