RAP widgets can be extensively styled to create a custom look & feel. This is done using CSS-syntax, so there is no need to touch any Java-code.
Theming can influence the layout by adding custom padding, define fonts, colors and images used by widgets, and even create rounded borders, gradients and animations.
You can see animations in this example in different places:
Using custom variants, single widgets can be styled differently from others, as demonstrated on this page. A custom variant is a name that can be attached to widgets in Java code and referred to in the CSS, just like classes in HTML.