The basis of using Cascading Style Sheets (CSS) to apply formatting to an HTML document is deceptively easy to understand, but hard to master. You define selectors that designate HTML elements, which contains formatting rules that apply to those elements and their children. If more than one rule applies to an element, they are applied using a priority order, from the more general to the more specific: that’s where the “cascading” part comes in.
It’s almost impossible to predict how a website will look by looking at the code. If you have to modify the CSS of an application you don’t know “just” to change a color or a font size using only your text editor, you’re out of luck. Figuring out the priority of each selector is complicated if you have many stylesheets that applies to a page, and you can easily miss a more general selector. You can’t just look at an element on the page and immediately deduce which CSS rules are affecting it, especially if there are many contradictory rules.
That’s where tools such as Firebug for Firefox or the Developer Tools in Chrome comes in. Among many other great features, they allow you to see which rules are affecting a particular element on the page, and to modify them on the fly.
Of course, it won’t modify your original CSS since it only changes the local copy in your browser, but you can quickly test what’s the impact on your page and then make the modifications. You can also use those tools to explore your favorite websites and see how they used CSS to format elements: it can be a great learning tool if you’re looking to grow your CSS knowledge.
Here is a screencast on how to use Firebug to figure out where the colors are defined on the Twitter website: