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."