CSS applies to HTML documents. A Forms application is a Java applet and has nothing to do with HTML stuff.
As Francois pointed out Oracle forms has nothing to do with html based technology. The way you can get the equivalent effect of creating consistancy in the way your objects are displayed is by creating a set of items you want and alter their apperance and behaviour and place them into object libraries (one library should do the trick). From there you subclass all of you forms items off these libraries.