What are some good free resources for learning web design?

At the risk of oversimplifying a complex field that people devote their lives to, what are some good resources for learning web design?

Between the two of us working on our company, neither is a designer (one developer, one product/business), and while we both have some passing understanding of design, it isn't our forte. We've considered buying a new website template but we largely like the one we have, and think the changes we need to make are less than 30% of the overall appearance.

Things I'm interested in learning:

• How to use design tools like Sketch
• How to make things look professional/polished
• Basic rules to be aware of (designing for different breakpoints, colorblindness/accessibility, how to match colors)

(I've considered hiring a freelance designer, and have looked into it over the past few months, but my designer friends I'd hire are not available for various reasons, and I don't want to devote the time to searching/onboarding/managing, etc.)