I changed my mind and I bought Charm Template for mixing the original HelixUltimate with CharmHelixUltimate for getting light/dark mode.
I think I am close to manage my aim.
In /plugins/system/charmhelixultimate/core/charmhelixultimate.php and /templates/charm/index.php
I see Charm:: that calls Class 'Charm'
Where is located 'Charm' class ?
Thank you for your help...