Rated 3 out of 5 stars
Does exactly what I hoped for, nice!
I would have really liked it if the icon styling would fit the rest of Thunderbird's black icons, rather than the colored ones that are added.
I really don't see why this add on needs another icon for adding a new event than the one there is already in the side bar for instance.
All toolbar buttons have been introduced into this add-on before similar buttons have been introduced into lightning. See them as a legacy ;)
I have no plans on changing the design of the buttons just because the thunderbird devs change their theme quite often. If you wish a consistent UI, you could install any theme bringing back the old thunderbird look and feel, as Ignore Aero, for instance.
If you want to adapt to the current theme, you may draw own button images and link them via userchrome.css. To do that, look into the chrome/skin folder within this extension (rename xpi to zip, then you should be able to browse its content), or contact me by mail and I can send you the css needed.