Joomla Theme/Skin Modification

I have an existing website based on the JA Opal theme. The theme allows choosing from 5 different colour schemes: red, blue, yellow, black, and athletic. I need to add a sixth colour combination to blend with a new initiative. The organization is hosting a display of the last 100 years of history of our organization, and the site will feature a number of historic photos in black and white. We will convert all photos to sepia tone and would like a complimentary website colour scheme. The task is to add another menu item to the list of available colors that will produce allow us to change to the new colour scheme in the same manner we change colours now. Usual CSS and XML changes required. If this experience is successful, we will create a number of additional colour schemes for upcoming events.