Update the .css for the Ref Guide

I put a custom .css into the Ref Guide before it was uploaded. I was going for something parallel to the Solr website, but only spent a little time with it. In terms of readibility of the text online, it's not great, which is putting it nicely. It's also very difficult to differentiate between "normal" text and monospaced text to indicate a command, program name, etc.

I'm attaching a new .css that can simply replace what's already in the Space Admin -> Stylesheet section. I did several things with this:

• cleaned up the .css generally, consolidated some repetitive sections, and added more comments in case future changes are desired.
• changed the font throughout to Helvetica, Arial, or sans-serif and updated the color to a slightly less strong black.
• changed the monospace font to match the font used in the code boxes (Consolas) and made them the same color as the text (default is a lot lighter).
• added a bit more space between lines.
• removed the negative margin in the header/breadcrumbs to give it a bit more space.