^ Yeap kh99, I've had to do that on a few custom styles now, I try to avoid it but due to how some CSS is called (Profiles in Dark Styles more so) it seems that is required on occasion.
You should be able to check the the forumid using:
Code:
<vb:if condition="$GLOBAL[forumid] == x">
Where x needs to be replaced w/ the forumid#
In some templates
Code:
<vb:if condition="$forum[forumid] == x">
Is not registered although
Code:
<vb:if condition="$thread[forumid] == x">
Would work in some instances however Global should always work

.
Edit: OR actually you could simply whip up a plugin using the parse_templates hook location to render this in headinclude_bottom... something to the effect of:
Code:
if ($vbulletin->userinfo['usergroupid'] == '1'){
$forumbg = '<style type="text/css">
html {
background: rgb(0, 0, 0) url(images/mybackgroundfolder/blackbackground1.jpg) center fixed;
overflow:scroll;
}
</style>';
} else {
$forum = '<style type="text/css">
html {
background: rgb(0, 0, 0) url(images/mybackgroundfolder/blackbackground2.jpg) center fixed;
overflow:scroll;
}
</style>';
}
$template_hook[headinclude_bottom_css] .= $forumbg;
That's a rough example, it starts and shows background1 to guests otherwise (it uses the terminology else) it shows background2 to everyone else... you could also adjust that part to check for forumid etc etc

. *Some adjustment will be required if you use this for example you may not want to use the css definition html, you may choose to overwrite something else instead and if you have any issues overwriting add in !important to the definition end

.