Wish I had the time!
Was going to suggest custom user fields used in conditionals to load different CSS... something like:
<xen:if is="{$visitor.customFields.font} == 'Tahoma'">
<xen:require css="tahoma-font.css" />
<xen:elseif is="{$visitor.customFields.font} == 'Comic Sans'" />
<xen:require css="comicsans-font.css" />
<xen:elseif is="{$visitor.customFields.font} == 'Arial" />
<xen:require css="arial-font.css" />
</xen:if>
etc.