Vinyl Music Player has a material design that is easy on the eyes.
It's easy to use and customizable (multiple themes including a dark one).
It supports folder browsing, sleep timer and a tag editor.
Licensed under GPLv3
Album cover: https://www.flickr.com/photos/vastateparksstaff/38645226714/ (CC BY 2.0)
Icon made by Nikita Golubev https://www.flaticon.com/authors/nikita-golubev from www.flaticon.com