Graham As it is still a beta the css is tagged on in a <style> tag which will be removed in the final version
I will ask about a 'own_css' option on monday.
Meanwhile if you want to play with it here is a copy of that css with every css property marked as !important so that it will override that <style> tag
http://syntax.xtgem.com/forum.css
you can upload it by 'remote server'
To make it refresh quicker I use a little xtscript to add a changing value on to the link address

<!--parser:xtscript-->var $v=<xt:tdate format="U" /><!--/parser:xtscript--> <link type="text/css" rel="stylesheet" href="/forum.css?v={_$$v|}" media="all"/>

Note some of the css classes may change in the final version of forum
here is a site where I have been playing with the css http://forum-template.xtgem.com/