Local sub-theme definitions - CSS
Define the CSS for any locally defined sub-themes. Sub-themes are used to create a custom appearance for certain types such as windows, buttons, tabs, accordions, sliders, switches, etc.
When CSS classes for the sub-theme have a name, the settings for that class can then be defined. This can be done either by hand or by clicking the 'Edit in CSS Builder' builder at the bottom of the dialog.
You can use the 'Design' tab in the builder to edit the CSS classes by simply adjusting style property settings. This makes it easy to set specific colors, fonts, amounts of padding, and so on.
Once you have defined a sub-theme for a given control type you should be able to select it in the 'Sub-theme' property for that control type on the UX controls page. For example, here is a spin control 'Sub-theme' property.