Rated 4 out of 5 stars

If there was an option to apply the Thunderbird Bright theme to the two titlebar buttons that launch Calendars and Tasks then this would be perfect, especially if used in combination with GlassMyBird

This review is for a previous version of the add-on (1.5.4). 

You can add CTBs toolbar button style to Lightnings 'calendar tab' and 'task tab' buttons on tabs toolbar. Go to CTBs options page an select the "classic style for tab toolbar buttons".