Is there a way to set active styles without user input?

[Lassebq]

E.g. through a shell script by modifying some file. Or running some js in the background (though preferably I want to trigger it outside of browser)

I'm using firefox extension, if that matters.

[Lassebq]

Okay so, let me put it differently
Is there a way to change active styles using userscripts/browser console?