An easy way of doing this on an existing web page, to see what locations are controlled by the classes or selectors, is to use the Firefox Browser and the Web Developer extension.
To use it load the page in Firefox, right click on the webpage, select web developer > Information > Display ID and Class Details. This will print small tags with the IDs controlling each section of the web page. Now find what you want to change, go to the CSS file, find the target attribute and make your changes.
You can also just open Theme.html or templates in a text editor, but I prefer the above method as I can see exactly what I want to change.
You can get Firefox browser Here.
The Web Developer Extension is Here.