297438 - stylesheet overlays for CSS

Status: NEW

(Core :: XUL, enhancement)

Description

[Karsten Düsterloh]

XUL provides a mechanism for overlaying XUL files with other XUL files and for
overlaying XUL files with additional CSS files, but we don't have the
possibility to overlay CSS files with CSS files.

Overlaying a CSS file styles.css with another CSS file overlay.css would just
work as if all the rules of overlay.css had been noted at the end of styles.css,
thus overruling rules with equal selectors as usual. 

The advantage will be that including styles.css now automatically contains the
overlay.css rules. This would allow eg. extensions to quickly make application
wide style changes without having to overlay each and every XUL file that
includes styles.css...