It's a nice little app, good attempt!

From a design perspective:

• Borders create a lot of harsh lines and it would be more pleasant to look at without them. The color differences should be strong enough to not need borders unless it's thematically required. In this case I think it would be better without them, so I would suggest just removing them.

• A different font for playlist titles versus descriptions would make it easier to read at a glance. A larger font size, as well, to create hierarchy.

• Some graphics - maybe a play button instead of the "play" word written out. You don't need the green checks and the red X's if you have a Play button that is clearly "enabled" or "disabled" depending on whether it's available via a source or not.

• The main function of the site, the player itself, should be more prominent and visible so I know where it is and how I can interact with it. Maybe a sidebar that is fixed position, or something similar. Think about where it could live on mobile, and then build up and out from there.

• Some playlist or album art would be great for visuals and would stand out well against the dark background. Consider using it as a visual element to display hierarchy and importance to the user through size and placement. Use it to show me the theme/mood of each playlist, but also use it to show me where the player is at, what is currently playing, what was just played and what will be played next, etc.

• Show me on each playlist how many songs are in that playlist, how long it is, and what genre(s) it falls under. I want to know more about it before I hit play and spend 10+ minutes listening to it.

It's a solid attempt! I hope my feedback is useful. :)