My skins all operate the same way. Choose the
Custom style, then go to
Mercury > Custom. You can choose one of the preset styles as the starting point if you like. Then you can create whatever color scheme you desire.
However, when it comes to the mouseover thumbnail caption text and background, I didn't include those particular design elements, since I couldn't imagine anyone wanting to change them. So, you'll have to hit
Mercury > Code > CSS and enter:
.justified-gallery > a > .jg-caption {background-color: #333333;}