Setting bg color via ^C,M does not work on all clients (e.g., irssi).
https://www.mirc.com/colors.html states:
"To specify a background color, a foreground color has to be given. So a
^C,8 attribute is not valid and thus ignored."
https://www.mirc.com/help/html/index.html?change_colors.html suggests:
"You can use color 99 for the default text/background color for a line."
chzchzchz