How to edit theme colors in TextMate 2?
Option 1: use TextMate 1:

open ~/Library/Application\ Support/Avian/Bundles/Themes.tmbundle/Themes/Custom.tmTheme -a TextMate
cp ~/Library/Application\ Support/TextMate/Themes/Custom.tmTheme \
~/Library/Application\ Support/Avian/Bundles/Themes.tmbundle/Themes/Custom.tmTheme


Option 2: edit the theme as an old-style plist from the bundle editor:



Option 3: edit the tmTheme file directly by opening it with mate:

mate ~/Library/Application\ Support/Avian/Bundles/Themes.tmbundle/Themes/Custom.tmTheme


The last two options require you to close and reopen files to make them use the edited theme. tell app "TextMate 2" to reload bundles does not work.

You can use pl and plutil to convert between the XML and old style formats:

pl -input file.plist -output file.plist
plutil -convert xml1 file.plist