With consideration for accessibility this depends on the simplicity of the layout. In order to achieve WCAG 2.0 AA compliance color combinations must achieve a

With consideration for accessibility this depends on the simplicity of the layout. In order to achieve WCAG 2.0 AA compliance color combinations must achieve at least a 4.5x luminosity ratio. That can mean different things may require color changes that were not required in the default scheme, as well as the opposite. As a result you really need files with different rules for that 20% that requires more than different color values.