- styling, or defining the visual elements like colors and fonts, and
- layout, or defining how the content is structured on the page.
Web developers who can also do design are so in-demand that they are commonly referred to as ‘unicorns’.
Be a unicorn!
Web design is a deep topic, but grasping the basics of CSS is essential regardless of what type of developer you become. Even if you only want to deal with the backend, you will have to grapple with CSS at some point and you don’t want to be clueless.
Learning CSS requires learning its basic concepts, such as how Selectors and the Box Model work. From there, working with it a lot will help you pick up and memorize the syntax of its various styling rules, like how to set the background.
To learn layout, you need to learn its various and finicky rules for positioning elements such as Floats, display and positioning, flex box, and/or grid.
Once you’ve covered the basics, you can learn various methodologies for writing maintainable css, and start using CSS frameworks like Bootstrap or Foundation.