Very nice icons and color schemes. I really like the gradients as well. Very good: the current email retains clear highlighting, thats a usability ++.
The Lightning subskin is particularly well done.
The only bad thing: the octagonal buttons and dropdowns don't look very clean (yet) - they look "Hand-drawn" and really stick out compared to the beautiful artwork on the icon sets. Also, the dropdown marker looks too much like a checkmark which is confusing. Without this flaw it would be a perfect 5.
This review is for a previous version of the add-on (1.6.1).