"Correct" Way to Add Custom CSS?
 

Not so much "correct," which is why it's in quotation marks, but what the majority of people do.

For example, the Folder Picker text on the private.php page isn't controlled by blockhead_color by default; it's controlled by something else that I don't remember now.

To make it be controlled by blockhead_color, I would do this:
However, I've seen some people that do things like this:

I always do it the first way I listed here.

What your talking about is "conventions" in CSS coding. Syntactically they are all correct to the parser. When starting off with CSS use your first method, the method you are using. After writing allot of CSS you find reasons to use the "long" method (method2) in some applications. It all depends on readability.

Seems like your on the right track :)

On a large project, good CSS planning is an art.

I find the "short" way (first example) much easier to read and follow when looking at hundreds of lines of CSS. Of course the only difference is readability and page size.