There's no built-in support for that, but this should do the job:
set chatFontSize=medium; color: white; background: black
You may want to change the first word of the setting to "small" or "large". A quit & restart of the program will be required for the change to take effect. Any changes in the Chat Options section of Preferences will undo the effect, so make sure they're all set as desired beforehand.
I make no guarantee that KoLmafia's own uses of color in the CLI window will remain readable with this change.