How can I change the color styles of a theme?

I'd like to be able to set the color styles shown on the pick list of a theme for elements in my slides. For example, each default theme in Keynote has different colors available when you click the drop down for an element on the slide, like text or shapes. Some of them are bright pastels, some are dark and earthy. There are 6 colors to choose from, plus gradients when appropriate. I'd like to set my own colors for a theme for the user to choose from. I know they can choose any color from the color wheel, but that's not what I want. I want to change the 6 preset colors for a theme.