Not a direct answer to your question but a hint how you can help yourself. On Linux GTKInspector is a very useful tool to explore the hierarchie and the properties of UI components. As a short example I use a slider for the parametric mask (input or output, sorry, in german, but I think you understand nevertheless).
This first screenshot above shows that we have a GtkDarktableGradientSlider. And we can take a look at the hierarchie :
Next we can look at the properties of the selected component :
and the CSS knots defining the properties of the selected component :
The tool offers a lot of further options. It enables you to find the name of a component and the location where it’s properties are defined.