Thanks..I found that out the hard way. However for anyone googling about this
the flex style explorer is the tool should have been inside flash builder, but is not.
It will help you visually style components and then copy the CSS selector into your program.
Yep it's can be a fine solution, but can't be used to skinning components from flash assets, which was a very good feature in the CSS design mode, and this is my problem now. I wasted a day to find out why my spark components doesn't work properly with my css file. It simply doesn't support the Embed(source="myFlash.swf", symbol="mySkinAsset") syntax in css, or I don't know. I would like to load skins runtime, I have a good solution, but it not workable with spark components. If you know a working way to loading spark skins runtime, please share with me, it could be a big help, otherwise I will use mx components.