The concept of our themes is that components always match each other, and to that end the themes use shared "components" that have the same classes and styling - for example headers, titlebars, buttons, etc.
The theme builder operates by using a few base variables and extracts further colors from them. It is up to you to choose the initial colors and you can preview the results on the right hand side. You can then manually tweak things for better precision.
If you want to implement tweaks per control, you can inspect the rendered HTML and see the available classes. Usually the component wrapping element has a class like k-treeview (for the treeview, for example) that you can cascade through. Most components have a Class attribute and if some doesn't, you can always cascade through a parent class from your layout.
If you start implementing CSS overrides per component, you may find the following useful:
UI for Blazor