This is really simple but amazing file structure design in Pure CSS that you can apply to an unordered list by Sazzad Hossain, this was created as a part of the BootAdmin template. You can see the parent-child folder structure and it also has open/closed folder icons, file icons and file sizes all neatly laid out.
If you are having trouble with the pen, try the archived copy on GitHub
Tree Structure Snippet
See the Pen File Tree CSS by Sazzad (@sazzad) on CodePen.default