First, there is a difference between themes and actually changing the look of the entire UI. Themes are purely for changing the look of the Aero MSStyle, which doesn't give you the ability to make any major changes. MSStyles are complete UI elements that are capable of changing almost every UI element of the computer. (E.G. Aero and Luna.)
However, there are no other "offical" MSStyles. Microsoft only supplies Aero, any others require patching the computer. You could just switch your Aero color to black and then it will appear darker. You also could switch to basic mode, and then manually change your windows theme.
Now, window blinds is a good solution, however be sure to pay for the full version first. Removing the trial version without upgrading is very difficult and caused some problems on my laptop. (Safemode, Administrator rights, all that jazz) They are a sneaky company. I'm not saying that their products are bad, just know that there is a possibility that it could cause some performance issues and removing the program is not any piece of chocolate cake.