Use bold colors or weights for Call to Action (CTA) buttons.

Visual hierarchy is the arrangement of elements in a way that implies importance. Twarog emphasizes using scale, color weight, and placement to guide the user's eye. The most important heading should be the largest.

If you are specifically looking at the PDF version, be aware that design is a visual medium best learned through video or interactive examples. A static PDF can sometimes fail to convey the nuance of animation or responsive resizing that his video courses handle better.

If you're interested in learning more about design for developers in general, here are some resources: