If you're referring to /ma on silent mod, go into settings when on a silent server and you can bind a key to it same as 'T', 'Y' and 'U' for public, team and fireteam chat. I have P for PMs and O for adminchat.
I see the same, except I see white background sometimes because I use the dark theme..
Shana is right, its rich formatting which the editor applies when someone copy pasted from the forums. Should simply disable background styles from the WYSIWYG editor IMO