I want the real colors to get a real reverse video style
The forum background colors are so close to white that it will barely be noticeable. The background color of your post will also change if a reply previous to yours in the thread is deleted or trashed by a moderator.
Still, the two colors used for the background of posts are #ECEDF3 and #F6F6F6.
To use these in reverse video style, you would usually just invert the brightness. This would change #ECEDF3 to #0C0C0D, and #F6F6F6 to #0A0A0A.
You can use these in BBCode with the usual color tag: color=#F6F6F6, for example.