My guess is that the system can't fully support using AERO. And when trying to change the theme it crashes out. What is your index score according to windows, as this will dictate what features will be enabled.
To get your index score do the following
If you have Computer on the desktop right click it and select properties. And in the window that will open you will see system and right under that is rating. Next to that will be the windows experience rating. With a range from 1.0 to 7.9.
If the computer Icon is not on the desktop click the Start Orb, and right click Computer and then click properties. And in the window that will open you will see system and right under that is rating. Next to that will be the windows experience rating. With a range from 1.0 to 7.9.
IF your system is 2.0 then aero will not fully work, it will be enable but many of the advanced features will be disabled. A 3.0 is needed for aero functions, although it may be hit and miss. The higher that number the better.
If your system is under a 3.0 or the box in which the number is grey, then Click on windows Experience Index, Which will show a more detailed score on your system. If it was grey it may run the Assessment to see what can operate.
There is no goal per say. Just what you want out of the system. Post back with the Experience Index Score and we can continue.
Jon