Thank you!
Here is a command line (to copy-paste and execute in a terminal) to do the whole process, not overwriting (just appending the new rules) profile-directory/chrome/userChrome.css and profile-directory/chrome/userContent.css if they already exist: $ chrome=~/.mozilla/abrowser/$(grep Path= ~/.mozilla/abrowser/profiles.ini | cut -d = -f 2)/chrome; mkdir -p $chrome; echo 'button, input[type="submit"], input[type="button"] {color: black;}' | tee -a $chrome/userChrome.css >> $chrome/userContent.css
