You can add any CSS style to a theme in the Stylesheet section. It allows a theme to also style third party widgets and to define CSS rules outside of the appearances.
Click the ... button next to rules in the property grid to show the code editor. In the code editor you can enter plain CSS rules. The image below shows a rule for the .qx-button class. It is applied to all Wisej buttons that don't have a particular active state.
We recommend to use the Stylesheet section only to style third-party widgets. If you use it to change the styles created by the theme system you risk breaking the appearance of the widget and for the CSS rules to be overridden by the theme system.