[xwiki-users] stylesheet override

Hi! I'm trying to override the css of my xwiki. I've been reading documents about making styleSheetExtensions (like MinimalStyleSheet Extension fro mhere: http://platform.xwiki.org/xwiki/bin/view/DevGuide/SkinExtensionsTutorial). I have to be doing something wrong because after creation StyleSheet extension like in this tutorial and setting "use this extension" to "always" nothing changes. additional I've seen that in Administration menu -> Look & feel -> presentation I've got button that "allow" me cutomize current skin. after pressing it and sayin that I want to customize it I've got a lot of empty boxes (style, header, footer). Always when I'm trying to write there anything the whole css crash and the site looks ugly. Please could you write, what is the simplest method to override some elements of the css of xwiki? I will be very thankfull! Wojtek