Background in Main.Activity is broken on colored page background

Here a light yellow background is used in a custom color schema.
The activity boxes in Main.Activity are rendered with a white background.

See http://www.pfarreiengemeinschaft-westum-loehndorf.de/xwiki2/bin/view/Main/Activity?viewer=changes between rev 4.2 to 4.3 for the fix gently provided by Sergiu Dumitriu.