this changes profile background colors to be dark, and sets up the groundwork for uploading user-selected background images for their profiles.  all the styling is in place; what remains is creating a new image uploader and (possibly) a new controller action.

for information on what command to use to darken the background image that's uploaded, see line 11 in `_profile.scss`.  the current image tests in the public folder are results of the convert command noted.