We have a problem in Ubuntu where Calendar is showing white corners.  We tried to fix this by having CSS in our theme which sets the GcalWindow background-color to transparent.  This worked in the inspector but not when this was in the our theme's CSS itself. (http://bazaar.launchpad.net/~ubuntu-art-pkg/ubuntu-themes/trunk/view/head:/Ambiance/gtk-3.0/apps/calendar.css)

I worked around this by 1st removing the .views section from data/theme/gtk-styles.css and then adding this to the bottom of that file:

GcalWindow  {
    background-color: transparent;
}

I then reapplied the .views CSS from gtk-styles.css in our own theme CSS (i.e. I moved it from gtk-styles.css to our own theme CSS).

This issue is also reported in Launchpad where I have attached a couple of simple patches which do the above:  https://bugs.launchpad.net/ubuntu-themes/+bug/1565267