What's the name that classifies this style (the black rounded lines)?
I've also made a variant without actually knowing what it's called.
There is not a specific word in my knowledge. But you can make it in very easy way, 1.Just select what shape you want to have strips on it 2.make two groups of strips with rounded corner one with the same color as your shape and other with 100% transparency 3. Now add colored strips with your shape and difference transparent strips from your shape
You could call this a "gooey line illustration". Not many relevant images come up if you search for that, but you can get a lot of examples and tutorials if you search "CSS gooey effect".
I don't think it has an official name in the way that "plaid" does, but "rounded lines pattern" will turn the pattern/style up more often than not.