@pauland, the OP's ask is not about post-processing, which would be nugatory, but about augmenting Xara's failings through CSS inclusions.

As Xara is mainly a vector program, it should generate code that displays at scale (e.g., SVG) and not rely on clunky graphical outputs for NavBars, rotated (transformed) text, carousels and the like. Xara has been improving its code gradually but its insistence on SmartShapes for Tables and SPANned text lines fails accessibility criteria. My day job requires that this is satisfied where possible.

Most of Xara's styling is controlled through external CSS files already. Most are quite small. ani.css, however, is 100kB as is jquery.js.
Xara has failed to use techniques that would use already cached jQuery and the version it presented is now quite stale (as is ani.css).

Most of my CSS & Javascript exploits are to overcome Xara limitations.

Acorn