Great in theory. Okay in practice. Unreliable in the long term. I've lost track of the number of times that large number of tabs get moved to a random group, or all moved into a single group. Sidebery was the initial reason I switched to FireFox/Zen, but I'm done with the extension now. Really disappointed.

Sidebery has ability to apply any custom CSS by the user, it's in the Sidebery Settings > Styles Editor. There is no need to distribute your own version of Sidebery — you should rather provide your CSS snippet separately, along with the brief instruction on how to apply it. Great work otherwise!