Find a Website's CSS
Because you can see every website's HTML, you can find where in the HTML they link to their CSS - and then you can see the CSS, too! Most often, the CSS is in a separate file and linked in the HTML like this:
<link rel="stylesheet" type="text/css" href="alinktothecssfile.css">
Right-click to view source and then in the HTML that opens in a new tab, look for this line. It will be nearer to the top than the bottom of the page. Here is the HTML for DIY's Skills Page, the link to our CSS is on line 31.
When you click on the link, it opens the CSS in a new tab.
Here is another example, on line 15, from Minecraft.net: