Works perfectly in Firefox, and also works quite well in Thunderbird, theming everfything except the email headers and the email itself. Lightning looks great!

Also works perfectly in Tree Style Tab if you use the plain theme and add the following in TST's options:
tab-item.active { background-color: #44475a; }

Good

Looks great! The accents from Dracula's color palette make the toolbar look really nice!

Need more update.