Twice this week i've been asked if Builder can override a site's global style sheet, specifically when they need to change a Heading 1 (for example) globally and they want to do this from Builder. I know we don't offer that yet, but just wanted to share since people are asking for it
I'll suggest to those that asked, but maybe more complicated than i'm imagining!
A possible work around for this would be to create a new section model, and then have someone on their team integrate that model into somewhere on their site that is on every page. Then they can create an entry in that new section model and add whatever custom css code they want. Then they can edit css and publish from that builder entry and it will apply site wide