Help finding documentation for having your application blend well with the components.

[GoHarder]

This question arose because it is possible to add some css to the window scrollbar.
The default browser scroll bar kind of clashes with the rounded design of the components.

That led me to look around to see if there was some documentation on how to have your bits of css like that blend well with the components in the library.

Is there anything like that already and I couldn't find it?
If it doesn't exist would there be something in the pipeline for that soon?
Or do I have to just wing it the best I can?