Sorry, I hadn't refreshed and didn't see your response before my second post.  I tried what you said.  I added
.blueevent .month_green_event_inner { 
background: blue; 
} 
to the month_green.css file, set the properties  CSSOnly = true; CssClassPrefix = month.green
In the BeforeEventRender event I set  e.CssClass = "blueevent";
Everything is still green (did a browser cache clear first),
This is what I see in the ie developer tools:
<div title="Beckmann - Private" class="month_green_event blueevent" style="left: 20%; top: 153px; width: 20%; height: 16px; overflow: hidden; position: absolute;" unselectable="on">
<div class="month_green_event_inner" unselectable="on">
Text - Beckmann - Private
Did I miss something?