There is a setting to disable it. It was in the beta change log but not the release change log.

Find your mirc.ini file via running this command in any window/channel/query:
Code
//echo -a $mircini
Close mIRC then open the mirc.ini in a text editor:
Add "darkmode=0" below "[text]"
Example:
Code
[text]
 darkmode=0
 ...
Save the mirc.ini file and exit the text editor.
Run mIRC.