Looking at the structure of a web site.
To check out the structure CSS, HTML on a specific area one can right click the area in Chrome, Firefox or Opera and select Inspect Element.
Here is a video from 2012 that I made to show how some of this works:
(The below video will be recreated making the process even easier to understand.)
On the left is the HTML and on the right is the CSS code.
Adjust the CSS on the right and notice the change in the browser.
Click the color, or click below the existing code to add additional code.
NB! Any change done in the browser is just temporary as it just shows you changes.
To make it permanent one needs to add the CSS code either directly into the style.css, or use a plugin such as Simple Custom CSS or Custom CSS in Jetpack.
If your having problems not seeing your updates then one thing to do is open Inspect Element then click the little jagged black wheel in the middle – located top right of the developer tools.
Click the checkbox for Disable cache (while DevTools is open).
Now you should be able to view your updates.