Changing Themes and Colors

Summary:

How to find the CSS command that needs to be changed for a particular element.

Content (required):

I have to rebrand our entire customer portal with different colors for nearly every major element. I'm wading through the class tags in the php pages to see what CSS tag to change, but then when I go to that tag, there often isn't a color attribute to change.

Is there some kind of guide available that explains the structure of the site.css and how to find attributes that need to be changed? Inspecting the element in Chrome is not really all that helpful as it appears that so much of things are inherited so tracing back to find where the color is set, for example, for the header or for <h2> is tedious at best and nearly impossible for other things.