Paid theme editing

If I need to edit the look of the paid theme, what is the best aproach to it? Only style without structure changes.

• To configure stylings from the admin area?
• To copy files and edit them?
• Somehow to create child theme with needed paid theme as a parent

For me, the best and modern approach would be the last one. But I don't know if you are able to do parent-child relations with custom theme.

Anyway, what way should I choose in order to suffer less later when I will need to support it?