AmigaSystem you are using for the buttons in the screenshot the same pattern as the one of the window one. Try using a different one. It should be with the MUI4 prefs changing the ones of "Buttons", "Text Buttons", -click on Look- and change "Background".
It doesn't work now. I really don't like at all the interferences of code in the MUI settings. If MUI3 can't handle those settings that the coder wants to get, then the code should detect if you are using MUI3 and make those changes, but not if you use MUI4.
To detect if you use MUI3 or MUI4 it is (I just asked Thore):
To check for the muimaster.library:
Version of muimaster.library.
V19.x is MUI 3.8
V20.x is MUI 4.0 in case x >= 5500 and MUI 3.9 otherwise
V21.x is MUI 5.0
From code it can be made this way he told me:
"Anyway, just use these macros with MUIMasterBase.
#define LIBVER(lib) ((struct Library *)lib)->lib_Version
#define LIBREV(lib) ((struct Library *)lib)->lib_Revision
#define VERSION_IS_AT_LEAST(ver, rev, minver, minrev) (((ver) > (minver)) || ((ver) == (minver) && (rev) == (minrev)) || ((ver) == (minver) && (rev) > (minrev)))
#define LIB_VERSION_IS_AT_LEAST(lib, minver, minrev) VERSION_IS_AT_LEAST(((struct Library *)(lib))->lib_Version, ((struct Library *)(lib))->lib_Revision, minver, minrev)