Further Sidebar Customization

I have found this layout amazing to use after two very minor customizations.  First, I believe changing the background of the sidebar leads to a more distinctive design.  Secondly, the transparency does not suit me.  I have added two variables setting the defaults in the base colors file so that they may be overridden in the custom colors file.

That said, I am not a designer and am unfamiliar with Sass.  I was not exactly sure where these settings should go.  But since my adjustments partially resolve #35, I thought I would share them in case anyone else was looking.