Textpattern CMS support forum
Re: txp:siteurl tag in styles (css) tab
smd_style allows this, among other things. That bit’s shamelessly stolen from ako_cssParse so if you don’t need the enhanced stylesheets switching / alternate stylesheets or real-time class switching ability (of the beta v0.30) then sniff out ako_cssParse and you should be all set.
Adds quite a bit of overhead to the processing so it’s probably not going to be a core thing unless someone can give a compelling reason why the speed penalty is beneficial (or they can make a patch fast enough that it doesn’t matter).