Currently it has improperly arranged borders: it doesn't look by now as "pressed", but rather still like raised - with the difference, that the light comes from "bottom right". IMHO the white "border" (bottom and right) should be removed completely, then it would look as "pressed" (checked on Linux, Tk8.6b2, all themes). Making bottom and right sides "lit" is wrong; either they should be as gray, as the background (or in any other bg colour), or they should be 1-pixel thick black line. How does it look presently: http://postimage.org/image/c68zo0xw/ (Upper: "untouched" (sets the associated variable to 0), the lower one: "depressed" - which, in fact, doesn't look as "pushed in". The "default" style - but the same problem is using all the other styles).
I would to add, that a very good example, how "depressed" button should look like, is the look of "ordinary" tk button, for example:
pack [button .b -image $someImage -relief flat -highlightthickness 0]
When clicking (and holding LMB), such button has proper "look & feel" of depressed button (properly "lit" still from top-left, and the image has been slightly moved diagonally in bottom-right direction). So there's not much more than copy the existing, available solution.