To create a Style for Nicecopier you must know CSS! Even better QCSS, the stylesheet for Qt.
For more information read qt-stylesheet
But basically its the same as css. You need to know the id or class.
In Qt the Name of the object is the Id
And the Type is the class
In the "image" you can hover over the elements and a popup will appear!
There you can see first the Name and then the Type
Download the sample Nice to see an example.
Upload Style file