How-To change the default cPanel theme from WHM ?
To change the default for new accounts, then can be done at WHM’s Basic Webhost Manager Setup interface (WHM >> Home >> Server Configuration >> Basic WebHost Manager® Setup)
To change the theme for existing accounts you can use WHM’s Edit a Package interface (WHM >> Home >> Packages >> Edit a Package) to edit the theme setting for the default package or other packages you have created. Making a change to a package will reapply the package settings to the accounts.
More information can be found on this at the documentation below: