tkinter.messagebox is based on tkinter.commondialog, first written in 1997. Since it works as originally designed, it does not seem to have been significantly altered since before themed widgets. Upgrading would be an enhancement for future versions, not a bugfix for all versions.

The SO answer is correct that tkinter does not build the messagebox, but indeed calls into tk with, for message boxes, the command "tk_messageBox", to get the builtin box.

Since I am converting IDLE on 3.6 to use ttk where appropriate, I would very mush like to be able to get the better looking versions of the builtin dialogs -- and in 3.6 -- and on all 3 majore systems, not just Windows.  Since tkinter still supports 8.4 (should this change?) and ttk requires 8.5, this needs to be an option or at least version dependent.

I report this issue in a comment on 'patthoyts' answer.