Background color in user cp (join date etc)

[BlessedFWI]

Do you mean the users profile or the user control panel? I am assuming you mean profile since that is where the join date and last activity is located.

If so, try this and see if this is what you are wanting:

Go to your adminCP and click on "Styles and Templates" > "Style Manager" > click "Go" on whatever style you are wanting to edit > Sidebar (you'll have to scroll down a ways) > Sidebar Background > then enter what color you want to change it to and save.

I hope that is what you are looking for.