Editor	Control Panel > Style & Colors > Forums Styles > Forum Category Background
CSS	.ForumCategoryHeader { color: #ffffff; font-size: 110%; background: #222222 url(); border: 0px; padding: 2px; margin: 1px; }

Editor	Control Panel > Style & Colors > Content Boxes > Even Rows Background Color
CSS	.AltGridRow { color:#ffffff; background:#222222 url(); }

The even rows (AltGridRow) effects both the topic listing and the forum messages. If you only want the individual posts to be separated, then use this CSS

CSS*	.MessageAuthorLinkCell1 { color:#ffffff; background:#222222 url(); } .MessageSubjectCell1 { color:#ffffff; background:#222222 url(); }

To modify the link color of that separator, add this:

CSS*	.MessageAuthorLinkCell1 a:link { color:#ffffff; } .MessageSubjectCell1 a:link { color:#ffffff; }

You can also add more styles to the links by copying the CSS above and changing the a:link to a:active, a:hover and a:visited, if you want to change those styles as well.

*Note: I only added the CSS here because I haven't checked the built in editor to see if it was possible to modify it there.