How to organize your CSS file

This is how I would do it. There are other ways. Just make sure that it is logical and clear to read.

  1. comments at top
    (also add comments to anything important)
  2. general definitions (HTML tags)
    1. start with Body
  3. id identifiers
    1. any fluid layout / grid layout -- keep together
    2. header, menu, content, footer
  4. class identifiers
    1. block/paragraph formatting
    2. text formatting

How to make a footer:

In CSS file

#footer {
  background: #111;
  color:#CCC;
  padding: 1% 0;
  margin-top:5px;
  /* for this webpage, I have to cancel the body margins */ 
  margin: 0px -6% -4% -12%;
}

.centered {
  display: block;
  text-align: center;
}

I'm not sure if .centered needs all of that stuff in it. Try eliminating some of it and see what happens.
On this page, the <body> tag sets the margins and it's hard to make the footer override it.

In HTML file

... 
... lots of stuff here ...
...
<div id="footer" class="centered">
    Created by Michael Harwood © 2015.
</div>

Look at the nice example of CSS at the top of this page: http://www.w3schools.com/html/html_css.asp