Be sure the theme name follows the " name _8x6.kth" or " name _10x7.kth" naming scheme. Notice the lowercase "x" between the size numbers . A space works in the name, but not after it.
Make your chart colors accessible for the user to reuse in other presentations. You can do this by including a special Keynote file with your theme that contains color chips, i.e., objects filled with your chart colors, so that users can copy the styles onto their own objects.