my attempt at adding a syncthing icon
Don't be too strict. I have not tested it as I don't know how to build it. Activities were taken from the syncthing-android source code (I hope it's nicer that with whatever apps you were talking about). I also add an SVG icon and a script I used to convert it. Note that my XXXDPI dimensions are different from what you've specified.
You also forgot to describe how to choose a color for the bottom 'shade'. Or to make it a semi-transparent thing like the top 'shade'.