Aqua is the name of the native MacOS X look.
Even with the help of tile, Tk applications do not yet look fully native out of the box. It is, however, possible to improve the look of the default widgets with careful settings. This page collects some of them:
Lars H: Does that address a different issue than that which is addressed by setting the -highlightbackground? Cf. discussion at the end of button.
Category GUI See also TclTkAquaBI