common: generate optimised css file for light

Strip whitespace and comments to reduce download size by 6kB (35%).
Silences site analysers such as Google PageSpeed about unminified CSS:

> Compacting CSS code can save many bytes of data and speed up download and
> parse times.
> Minifying http://sheet.shiar.nl/base.css could save 2KiB (41% reduction)
> after compression.

Tried a couple of existing tools [from Debian] but they mostly failed to
parse more complex rules, or tried to make unwanted "improvements".
Rather write some simple regexes to specifically and transparently replace
whitespace and comments.