Can I design some simple stuff by using CSS on it? Like adding a background or something? All the videos I saw is just plain and default. Or this what you are talking about: SFGUI is WIP?
Style language they got is not exactly css but it's extremely close to it, maybe even fully compatible.
You can change colors, fonts, per widget type, per (css) class, per (css) id, for all widgets, for all widgets that have certain parent class etc.
You can change it for states, so prelight button, pressed button, normal button etc. all have different styles.
But you can't have round corners, fancy animations or texturing of widgets, there was project(Bob) to create rendering engine that'd allow that but it kind of died.