Custom CSS for WebUI

Hi,

is it possible to customize any node in the WebUI of RTC with a Custom Theme including a CSS as it is descibed here -> Theming

I've already uploaded a zip containing a css that tries to modify the WebUI and activated it. But it seems that it is not included when i open the page i tried to modify.

here is the content of the css(should modify the wikimarkup tables, the first 2 rules were for testing only):

.detailsSplitTableWrapper {
    background-color: lightblue;
}

.default_WTH .odd td {
    background-color: lightgray;
}
.default_WTH th {
    background-color: lightblue;
    border: black solid 1px;
}
.default_WTH,.default_WTH td,.default_WTH th {  
    border: black solid 1px;
    border-collapse: collapse;
    padding: 5px;
}


Is it even possible to modify these elements? Or have i done something wrong?

Thanks for your Answers.

ps: The used Version is 3.0, but we get 4.0 in late October.