Licencing depends on the licence of the parent theme.
To answer 'In the document, I also found a point that "Most theme developers simply add a few changes to their new theme by basing it on an existing one" so can you also explain this point' - which document did you read? If in the Moodle docs, then there should be a history that identifies when that was added and by whom, then you're in a position to ask them.
To answer 'In the document, I also found a point that "Most theme developers simply add a few changes to their new theme by basing it on an existing one" so can you also explain this point' - which document did you read? If in the Moodle docs, then there should be a history that identifies when that was added and by whom, then you're in a position to ask them.