Currently the thumbnails that represent Blend files are low-res, mostly monochrome, and hard to quickly understand.

This patch fixes one mistake. All of our thumbnails are stored at 256x256 PNGs, but blend thumbnails are actually created at 256, then reduced to 128 and then increased back to 256, making them blurrier than they need to be. This patch also removes the odd dotted frame and darkened portions. And then it shades the thumbnail using random colors (specular highlights, cavity, outlines, etc).

The following image illustrates the changes. The top row shows how the sample blend files look now as thumbnails. The bottom are versions that should be much easier to understand:

Using "random" colors for objects is deliberate here. Many blends could be given thumbnails that use the actual materials and textures, but then that process takes a very long time every time you save. Doing it this way is very fast. It also works well regardless of the state of your materials.

The following shows what my current File Manager (UI scale 1.2, large thumbnails) looks when viewing some sample blend files:

The following is after this patch is applied, along with D9397: