A fresh coat of paint, clean lines, and gentle curves. These are not three separate ideas. This 'concept' does not aim to rethink the Finder, or the system as a whole. Instead, it offers a look at a simple visual update of today's macOS.
Rounded corners and the elimination of borders around components aim to soften the interface whilst a drop of colour has been added in the sidebar to help give context.
Files and folders are easily distinguishable by thinner tags, and the expand/collapse folder button has been removed. Instead, simply click on the folder icon to reveal or hide its contents.