How do I learn to code in HTML and CSS on my Mac? Web design is one of the most useful skills you can learn, and it’s easy to learn HTML and CSS (the two main pillars of web pages) in Mac OS X.

Click the scroll-down arrow in the middle column and select "Add CSS." Enter your custom CSS code in the "Add Custom CSS" box. Click the "Apply to Blog" button located in the right corner of the page.