Isn’t this what you describe?
Side note: Window managers are only responsible for the window decoration, i.e. the frame around the contents and the title bar. Not a lot to tweak there, but most window managers do actually allow changing pretty much every aspect of it. GTK/Qt are toolkits, not window managers.
I'm thinking of something where say I load up a program UI Tweak, I don't need to know anything about the software I'm working with, I just select the element that I want to adjust, the label sizes for text in the dialogue, or the colour of the text by clicking on it.
Think Autohotkey's spy tool[0], when using Autohotkey with the spy tool, I just select the elements that I want to manipulate and then choose what I want to do with them. In my opinion, this significantly lowers barriers to entry for users wanting to change things to suit them. Then the developers of such software can work out ways of making apps work better with such a system. Maybe linux based UI apps need to improve their api so that they can take instructions to adjust it? Or they just add some simple hooks the UI Tweak can understand and use to adjust behaviour?
I don't know what users will want, but allowing users to describe what they want, letting them know it's possible, and giving them an avenue to share and communicate their needs can only help improve the quality of the software, as well as provide an avenue for devs that care about user experience to have a channel to understand what needs might not be being met by existing offerings.
- [0] Image example here => https://stackoverflow.com/a/45680175
I never went that way as I prefer to just use a theme designed by someone who knew what they were doing.