Post-process the CSS so excessive duplication/size is avoided

[sinclairnick]

Currently, the output CSS is not optimised, resulting in larger-than-necessary output files. There are likely various avenues for improving CSS minification, such as just deduplicating common styles and joining selectors etc.

[jokull]

Lightning CSS could be a good bet.

[sinclairnick]

@jokull I'll consider this for sure