(bug 44141) Vector: Phase out one-pixel images.

(bug 44141) Vector: Phase out one-pixel images.

Replace background-image declarations with background or border.
Interestingly some of the elements already had a background-color
set (different from the color in the image background).
Those colors were fixed.

Additionally, move the borders of #content from three different elements to #content itself.

Removed the now-unused files as well. preferences-edge.png was already unused before my changes.

Change-Id: I0a1fed69e05cd05ee6dae54aaca8e7f5dc837723