I was looking for something like this for hours and I come across this module. At first I didn't know why it didn't work but at last I solved the problem with help of Christian Ludwig.
I had problems because:
- My joomla was installed in the subdirectory and I didn't set the path properly
- From the templates I didn't remove the path to original css file.

Now it let me change anything on the website I want. Changing top image or colors now isn't a problem. Using it is very easy.
I'm very pleased especially of help of Christian Ludwig.
Thank you.