Editing a theme


I am relatively new to Drupal and working with Drupal 8.

I have downloaded some themes for some sites I am working on and I want to know if there is any restriction to how I can adapt them.

Can I edit a theme anyway that I like or are there restrictions on what you can do to change a theme?

I have a clear idea of what I want the pages to look like so I was planning on getting a theme that was the closest and then editing it until it is exactly right. Is that what developers generally do or do they start from scratch? Are there some starting themes that are particularly good for editing purposes?

Thanks for any replies.

Drupal version: 

Source: https://www.drupal.org/taxonomy/term/2/feed