Until now, you have only learned to extend WordPress features using plugins. It’s time for you to jump into theme development, another equally important part of WordPress development.
Themes give WordPress sites a proper appearance. It defines the structure of the website and the stylesheet. Without a good theme, any website will look like a collection of static HTML pages without any style applied to its elements.
Themes also greatly influence the user experience (UX) on the site. A good theme can do wonders for business while a bad theme can even kill your organic search engine ranking.
By the way, don’t forget to check out the Theme Developer Handbook.