If you've found any issues, feel free to report it in this thread.
Download & Known Issues:
https://github.com/FranklinDM/Milestone ... ag/1.0.0a1
This is possible to some degree if you'd do it yourself but it will only affect the main toolbar background. You can define the (--toolbar-custom-color) variable with (-moz-Dialog) using a new style in Stylem or through the use of your userChrome.css file. This isn't possible with the current test release though, as I haven't fixed up the usage of that variable yet.