What I'm looking for is:
-Allow to choose theme for the window decorations (titlebar etc...) and the desktop itself in its settings and have this applied consistently in the desktop (all applications use same window decorations on the outer part of their windows, ...).
-Let application have their own icons inside of the window (I have nothing against applications reusing icons, but let the developer choose them, not desktop-dependent since that's clearly not working out). Like say the "save file" icon in a text editor: I'd rather have a well-chosen one by the developer of this text editor, than one that either doesn't work or look completely out of place but was enforced by some desktop standard.
What I don't like is:
-applications changing / removing the outer window borders. They have no business changing that, only the inside content of their windows. E.g. a browser removing the title bar of its window can make some menus the desktop provides there inaccessible, make it invisible which window is focused, etc...
-application using external theme (e.g. based on some desktop standard) icons for its internal contents and then not working properly because the creator of the application could not know what the external theme would look like. This does not work out for various reasons: having 1 desktop setting makes it impossible to fix individual programs since changing theme to fix 1 may break another, e.g. say you have themes "adwaita" and "greybird" in your desktop setting, theme adwaita breaks program A (e.g. it shows black text on black background; just arbitrary example, not saying adwaita actually does this but I know at least one epub reader that showed black text on black background for some desktop settings in real life) but works in program B, and theme greybord works in program A but breaks program B (e.g. its icons will be invisible or whatever): If program A and program B just had their own inner contents, this problem would not exist.
Also, different applications having to be consistent across each other for their inner contents is something I couldn't care less about, so it's not worth all the problems it causes.