There are two /**** Event Module - START ****/ sections in App_Themes\Default\Default.css. Some of the settings within these sections are duplicated so if you find and change the first one, it then gets overwritten by the second one in the same file. I doubt this was intended. I suggest that the two sections are combined into one section in order to avoid any confusion.
I have found the duplicates in MonoX v4.5.3206.40 [11/11/2011] and MonoX v184.108.40.20674 [4/13/2012].