If you are looking for online resources to learn CSS, you’ll find all sorts of options. Simply search ‘learn CSS and you’ll find tons of free tutorials as well as snippets of CSS, many of them even give visual examples of what the code will do.
Here are a couple of resources to get you started:
Adding CSS to your WordPress Website
Lots of themes built in the last couple years, have a place to add custom CSS built right in. If your theme does not have this option, there are several plugins that you can use to add custom CSS, just do a search from your plugins page.
If the changes you want to make to your site are more involved than just a few snippets of code, you may want to create a child theme. Creating a child theme in WordPress is pretty straightforward, here are a couple sets of instructions:
Hint: you never want to edit your theme files directly because you will lose your changes when the theme gets updated.