Dear sir,
From your project description, seems that only color codes have to be changed in template css file. That is easy, but often color it is not only in CSS, but in other files...that requers more time.
I am willing to do a job for a price of an hour, but it may take more than that...couple of hours...when colors are changed, then you approve or has to be changed again..etc.
Regards,
Milos