Support question - which file do I edit to add class to a block?

Maybe my other ticket can be closed as user error.

Can you please please help me with two hopefully simple questions?

1. If I want to override the $Primary color with a new one, what file should I edit so that the gulp-watch picks up the change?
2. If I want to create/tweak a block class such as adding a padding or margin to class "block-views-blocknews-most-recent-news-block" where would I make that edit?

I have the basic layout of my page worked out but need to tweak some look and feel items for it to be complete and I am stumped.

I don't think I want to be editing twigs because they just seem to let you tweak the html structure and add classes. But where should those added classes be defined?

Thank you in advance for anyone who can help.
Noob!