Once you have an understanding of the CSS basics, getting familiar with the files that make up your theme is the next step.
We've mentioned this before, but it bears repeating that there are no "pages" (in the sense of a single static, tables-based HTML page) in a PHP-driven site like WordPress. All of the data that you add to WordPress (including posts, pages, comments, and users ) is added to the MySQL database. The data is filtered through a theme along with the help of plugins to create a website with different pages. But each page is made up of several different pieces. Within a theme, common elements (such as the header, footer, and sidebar) exist as separate PHP template files. These pieces are combined and served based on the theme structure and according to actions taken by a user (such as clicking or searching).
On the front end, your users are able to navigate to (and bookmark) URLs that seem a lot like pages. However, hopefully...