Technically, this is what Mozilla calls a dynamic theme. It is clearly stated both on the Preferences and the Usage page (link to the Usage page is in the Preferences page) the problem with using this addon and themes at the same time.

Actually, it can work even without enabling the titlebar. The layout is different, since the window min/max/close buttons are moved within the actual window in the upper right, since there's no titlebar. To handle this, there is a setting in the Preferences for Titlebar enabled or disabled. Again, this is covered in the Usage page, along with screenshots.

There actually is no switching profiles, but rather it starts another instance with the desired profile, which you can get to quickly by opening a new tab and entering "about:profiles" into the address bar.