If you're using one of the templates that come with DVDA, you can locate the template files on your hard drive and edit them -- searching should find some earlier threads on this.

If you want to do a custom highlight, there are several threads and there's an explanation in the DVDA pdf manual. As briefly as possible, you import 1-3 shapes for every button highlight, set the colors, and then place an empty image-type button so that its frame encloses your imported highlight.

Why 1-3 shapes? You can set colors for the fill, outline, and anti-alias (or blend) shape for each highlight -- typically (i.e. subtitles etc.) the anti-alias surrounds the outline, which itself surrounds the basic fill shape. You can specify different shapes as simply as fading (feathering) your B & W graphic with DVDA set to transparency, or you can get more involved. You can use simple png files, or go thru the hassle required and use a multi-layered psd. [personally I find it quicker to turn off layers I don't want in P/Shop and export the visible to png.]