Very nice theme. It hurts my eyes after looking at it for to long. As previously stated the transparency level is down to a point where the sun becomes the main focus of attention over your categories and forums.
I can see why you would describe it as cool, because it is quite graphical and unique. However, to be honest I wouldn't be inclined to join a forum with a theme like that, for the simple reason that it is uncomfortable to read the text.
I don't think it's the transparency as much as it's the sun in the center messing things up. If that was removed from the image/toned down to about Saturn's color intensity, that would go a long way in improving the look. It's not sized correctly in proportion to the planets anyway (it should be much larger. Also, earth should be smaller. People don't realize how truly BIG the sun is), so you wouldn't be losing anything. Also, I'd change the thread activity icons to something more like a planet or comet. The rose-shaped-sun look would be more appropriate for weather-related forums than astronomy forums.
The buttons for "new thread"/"reply" - the teal reads fine, but I feel like that's the wrong color for this theme. It's too warm. Something in a purple direction would probably make more sense.
Oh yeah, and definitely get rid of the rotating Enterprise gif. That's just silly 😉
I have always hate transparent forum with a bogus background. I have seen myspace layout like that being submitted to a myspace layout site. I have no clue how it became popular as it always hurt my eyes.