Rated 2 out of 5 stars
+ The good part:
As for me, the add-on works and is helps making sure which account you use.
+ The bad part:
You only get to choose background colours from a palette of fade colours and for me it was difficult to find a few which were both distinguishable and not confusing (no, I'm not colour-blind but orange and red would look too alarming for me, leaves green-ish and blue-ish).
+ The really bad part:
The add-on makes Thunderbird display "deactivated" text in the background colour (i.e. invisible), this can be mitigated _partly_ by forcing the use of the set-up foreground colour. This means when you read an email, you will no longer see the sender, subject etc. in the thread pane. Also, button texts disappear when you hover with the mouse pointer. This took all the fun out of using the add-on for me.
Note: I am using Linux, the issue(s) may be plattform-specific-