Go to: "C:\Program Files (x86)\Atomia\AdminPanel\Themes"

   - duplicate 'Default' folder with different name (say 'NewDefault')
   - change stuff inside 'NewDefault' <- whatever
   - be sure to change name of this folder from 'Default' to 'NewDefault' (or the name of your theme)
     * this one:
       C:\Program Files (x86)\Atomia\AdminPanel\Themes\NewDefault\Content\css\Default
     * should be like:
       C:\Program Files (x86)\Atomia\AdminPanel\Themes\NewDefault\Content\css\NewDefault

   As opposed to HCP, BCP, you have no problem with copying whole content of the theme from Default to NewDefault,
   and you don't need that parentTheme attribute at all. It works like a charm.