This shelf bracket system allows you to change the cover plate at will. 
The cover plate hides the screws of the bracket base.

I have included the STEP file for a blank bracket cover so you can create any cover you want.

You probably want to print it sideways with supports for strength.

Usage:
Take a look at the Bracket Base, one leg has 2 clips on it, the other leg has angle sides.
The angled sides leg is the top. 
1. Use screws to mount the side with clips on it of Bracket Base to the wall.
(repeat this step for each Bracket Base you need to install)
2. Use screws to attach the shelf to the Bracket Base, the side with angles.
(KEEP THE SHELF SUPPORTED UNTIL THE COVER IS ATTACHED)
3. Slide the Bracket Cover of your choice onto the Bracket Base. The top angles should aline easily.
4. Push the Bracket Cover in until the Bracket Base clips onto it.

NOTE: The shelf bracket cannot hold much weight, you MUST use a cover to hold the shelf up.
when changing out covers, be sure to support the shelf while the cover is off.