Hi, Firefox user 14902526! I’m glad you like the add-on.

I’m sorry that the wrong colour is being picked in Gmail. There is a workaround: on the add-on’s options page (via ATBC pop-up → More Settings…), open “Site list”. Add a rule for “mail.google.com”, select “pick colour from element”, and type in “.wl” in the textbox on the right. This should allow the Gmail theme to be detected.

In future versions, improved colour picking methods will be applied. So, stay tuned!