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xoops is a dynamic system and html output is parsed by a templategine. If you are able to set a var for your requests it would be possible without too many hacks.
when your theme is a general theme for main cats and you want to switch images, colours on some pages, use different stylesheets for different modules or pages
A possible pagevar might be the title of the actual page but that would be too long, ..., use <{$xoops_pagetitle|truncate:5:""}>
If your page isn't that css friend use <{$xoops_pagetitle|truncate:5:""}> to catch pages and create querys to display the wanted html
<{if $xoops_pagetitle|truncate:5:"ddddd"}> here the html ....
when you can identify a page you can act when it is displayed
but you should read into the topic though. With XOOPS everything is possible, frontieres are only in our heads.
happy xoopsing