I've recently updated to Mojave and I noticed that a TextMate theme I use called iPlastic doesn't allow a transparency for the background. For the same background setting in High Sierra the background now is solid. I've tried modifying the theme directly in the Bundle editor and tried different color codes for the background entry, which was

background = '#EEEEEEEB';

before the OS update, but I haven't been able to add any transparency. Does anyone have suggestions?