@atomicpoet Not much if you compare the UI, I've just selected each icon myself and done a bit of polishing here and there.
The main difference: Elk is a separate web app in separate URL where everything has been built from the ground up using Mastodon API.
Mine is just a CSS file on top of the original Mastodon. I like the polished functionality in the back end of the default Mastodon, I just wanted to make it look a bit better.