Hi, I'm the creator of that theme.
There is currently no way of adjusting the transparency there through VSB -- you would have to recreate the graphics to have a more transparent base. The reason why it is dark is because at the base layer of the graphic, I added about 50% (transparent) black. This is not controlled by the MSSTYLES, but rather by the graphic used.
On Vista, I created the theme to partially reflect the feel of Vista (dark Taskbar/Start Menu).
I decided to keep the same look for the port because I did not know how people would react to it.
Perhaps people liked my theme because of the dark taskbar? I just played it safe and did what was expected in a port -- retain much of the graphics and feel.
Anyway, I intend on creating a lighter Start Menu and Superbar to better mimic the default Windows 7 Aero visual style. When that will happen, I do not know, so in the meantime, if you have VSB, can recreate graphics, and have the time, feel free to mod it to your liking