I have a topbar, i.e. something like Facebook or or StackExchange or Twitter's top portion of the screen, and I want it to have a different background than the rest of the page (the stuff below/the main body). How do I accomplish this?

You have to place the your header_block outside your wrapper

 <div id="header_block">
      ...... Header_block Contents
    </div>
   <div id="wrapper">
      ...... Entire Page
    </div>

css

   #header_block
     {
           position:fixed;
           width:100%;
           float:left;
      }

This should do the work

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