Extending : CSS Support for XUI Editor Extension : Extending customized XUI styling
  
Extending customized XUI styling
Extension developers can customize and extend their own XUI styling for widgets if they require more vivid styling out of UNICOM® Digital Transformation Toolkit (UDTT™) default scope.
You can refer to the tooling part from the sample:
BTTExtensionSample.zip
See also
XUI styling configuration
XUI styling mapping
XUI styling interpreter
CSS Support for XUI Editor Extension