David, the forums do NOT support skinning. They simply have an interface that can't just be accomodated by the core CSS Classes. Because of this, we tried to make it easier for people to create/adjust a theme to more match their site as need be. Anyone should of course expect changes in the future through the use of new images and new css classes (dnn core ads icons all the time). It is our assumption that you as the site developer will test this type of stuff before doing it in production to update your custom themes accordingly.

I have no idea how many people use customized themes, from what I have observed over the years it appears to be very few. I honestly haven't kept a log of what changed here, however, you should easily be able to use Beyond Compare of old theme against the new theme (css file) as well as the images folder to see the new images.