PoC: show how greenblaze terribly impacts performance

The greenblaze theme currently use a lot of embedded images. There is one for the background, obviously, but each item uses one, for an object that seems quite easy to do in pure SVG: a rectangle with a gradient.

In this commit, I removed all these images, keeping only the background. Since I have absolutely no SVG nor inkscape skills, I could not replace them with proper objects. But the visual impact on the theme is almost non-existing, while the performance impact is surprising: startup time goes from 1.6 to 0.3 seconds on my computer with this change.