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 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.
2 jobs for !24 with work/ducroquet/lighter-greenblaze in 1 minute (queued for 40 seconds)
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