I got this to work for background color. However, I couldn't get a tan at all with the custom colors; best I could do was a gray. Also, when I changed it to gray, then I lost all the envelopes (and I think the green arrows). Will have to experiment some more.
I *think* that yesterday, when I customized the My Profile page and changed the font to medium, that today somehow the board implemented that change on the posts. At least, the font seems a lot bigger and darker to me today than it did yesterday. If it did implement font, it did not implement the background color for the posts (but may have implemented the color I choose as the sidebars to the posting page).
sigh..... Guess it will just take some time and experimentation to get everything right for me.....