I just spent hours perfecting a subtle design for some "tab" buttons, and got them just right. However, now when I click on the button in browse mode, it highlights the entire button as black for an instant while I click it. Is there any way to control what color this highlighting is (or in fact to get rid of it?)

You cannot control the highlight colour, nor turn it off.

If the whole rectangle of the image highlights even though the "button" is a non-rectangular shape then you need to look into adding transparency to the image. There were IIRC recent posts about this to the Forums.

There are also demo files downloadable from www.databasepros.com that demonstrate other techniques.