Categories - Custom Background Colours (To Each ID)?

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Forums are displayed with the 'forumbit' templates but already have IDs:

<li id="forum15" class="forumbit_post L2">
  stuff and stuff	
</li>

All you'd need to do is to go into your additional.css or forumhome.css and add a class for that ID. In the above case it would be:

li#forum15 {
    background: white;
}