Tk has basically two sets of widgets, the classic widgets and the themed widgets. The classic widgets are default because of the need to support old applications, but new code should use the themed widgets. (They can't be overloaded because they work in quite different ways, and there's a lot of code out there that depends on the old classic way. Alas.) With the right theme, the themed widgets (typically referred to as Ttk) look native and behave natively on Windows and OSX.
There are partial Ttk themes that delegate to Gtk and Qt available (this presentation from last year shows them in action) but they're partial, in large part because of the poor quality of the themes (Gtk/Qt themes, not Ttk themes; this is confusing!) in typical use with those toolkits. The major problem is apparently in the reporting of metrics, which often seems to be wrong (resulting in widgets getting the wrong size or visual elements being put in the wrong place); I understand from talking to the author of that paper that a significant fraction of themes only happen to work with Gtk/Qt, but cannot verify this from personal experience. (I'm on OSX so I use Ttk's direct native support.)
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