Don't worry, maybe I wasn't 100% polite ( English it's not my mother tongue ) but wasn't my desire. No man, you can't use css to change standard TW Widgets.
Someone shared with me an extension which allow css changing, but I won't recommend using it neither I used it. TW Mashups are good as they are , playing around with css will break Composer easy to build interfaces also migration between TW versions will be a pain. As said before, if you want to build something super cool, better you stick out of TW Composer Mashups.
Another option it's to build you own widgets, as TW it's extensible and you can build anything you want on top of it, and you will have the easy mashup editing that brings Composer.