What you need to do is:
Click on the theme you'd like to start with.
Click on the EDIT icon (next to the DELETE / trash icon) to open that theme.
Select the color you want by clicking on it.
Press the SET AS BASE COLOR selector above that color.
Save the theme with a different name so you don't mess up a wanted theme.
Now you can manipulate the colors via the Kuler rules or color sliders all you want. Just save the new theme when finished.
Thanks, so much!
You're very welcome.