Welcome to ZK, the easiest way to build modern Java web applications.
ZK Style Customization Guide describes the styling concept and design of ZK components. For installation, please refer to ZK Installation Guide, for concepts, please refer to ZK Essentials. For a full description of component properties and methods please visit ZK Developer's Reference.
ZK Themes is an open source project that has a collection of various themes, including breeze, silvertail and sapphire. You could derive your own theme from any of them and it is also a good source to know how a custom theme works.
For more details about customize theme and how to apply customize CSS, please refer to Theming and Styling.