Tk Source Code

The header background of ttk::treeview widgets is configurable using the Heading style:

::ttk::style configure Heading -background $color

and this setting is effective for Ttk themes such as default:

But the aqua Ttk theme (used when Tk Aqua is started) does not use this setting, at least on macOS 10.9 and later. Depending on whether the toplevel is using dark mode, either DrawListHeader() or DrawDarkListHeader() (ttkMacOSXTheme.c) is used. DrawListHeader() draws the header background using hardcoded colors: #EEEEEE if the toplevel is active, otherwise #F6F6F6:

DrawDarkListHeader() doesn't draw a background fill color at all, so header background is effectively the treeview -fieldbackground color (which is systemTextBackgroundColor by default):

TIP #584 is now merged in main, and could help since it provides new introspection capabilities in ttk.

X-ref: [b46642fe3b], similar discussion.

I would say that the bug here is that the -fieldbackground color should not
be used in Dark Mode.  The native equivalent, which I think is NSTableView,
does not offer a choice of header color, so Ttk shouldn't either.

I am not very familiar with how Ttk C internals work, so it is not immediately apparent to me how this can be addressed. Maybe someone more familiar with it can give this a try.