CSS Variables or Custom Properties have been around for a bit. This walk-through shows a ton of different ways you can start working with them and how to work them into our web apps now.
This one is fun to go through if you have been writing CSS for awhile.
Self Hosted Tools
I have never been one to ever want to self host a tool or some important data. But, Notea has a setup that is interesting you can host your files on S3 and host it on Netlify.
Tiny Helpful Things