Upgrading to Thunderbird 15 has caused problem in the display of toolbar

This great theme (Greybird) has been working perfectly under Ubuntu 12.04 until I recently upgraded to Thunderbird when I got a light grey stripe between the titlebar and the toolbar in the main window.

Toolbars in the main and compose windows seem to behave differently whereas the toolbar in main window (unlike the compose window) is not displayed with the same background color as toolbars elsewhere. I used to correct this by inserting the following in the file "~/.thunderbird/profile/chrome/userChrome.css":

#menubar, toolbar {
background-color: #cecece !important;
}

Now I have the option of either having the toolbar with the same background color as the titlebar (as above) but with a lighter grey stripe in between, or removing the custom setting above and having the entire area in question with that light grey background.

Might this be due to a minor bug in the theme or do I need to do something extra in my custom Thunderbird settings?