Look the body classes of the forum pages.

The "set document key" plugin add some unique classes like "forum_ctrl_index_index"

Use this class in your css.

See, that's what I attempted to do but it's not working for me. 

I'm currently trying:

 body:not (forum_ctrl_index_index) .ow_alt2, tr.ow_alt2 td {   background-color: #23272a}

+1 Norias

body:not(.forum_ctrl_index_index) 

The "not" word is used when you want all pages but not forum to show that custom css.

That is wrong, also please take special attention to spaces and dots, do not add spaces if it is not needed or it wont work. A dot means it is classname, you can use "#thistagId" instead of dots in css when you want apply css to a tag with id="thistagId"

Try this:

 body.forum_ctrl_index_index tr.ow_alt2 td {   background-color: #23272a}

This selector apply the css to all table rows which has class "ow_alt2" on all pages where body has class "forum_ctrl_index_index"

Senior Developer.

It did occur to me some time later after this post I had the logic wrong, but was still unable to work it out.

Thank you so much Senior Developer!