Learn flexbox. Learn the box model (especially when to use margin vs padding). Learn how style cascading works (and why we use SCSS and component development to mostly avoid thinking about it). Learn media queries and how to target different screen sizes. Most of all, don't ever feel badly about doing a web search “how to do x in css”, the spec is huge with tons to remember and you won’t memorize everything without using it regularly for an extended period of time.