The board is designed for film capacitors at C2 and C7 hence the square silkscreen outline.
All points on the board with silk-screened circles are electrolytic.
The LED silkscreen shows a flat side which is the cathode. Remember FLAT FAT CAT = CAThode
This information is identical for all GuitarPCB Builds.
You can use electrolytic but we prefer to use MLCC or Film capacitors when that part of the circuit is in the Audio Path. There is a good deal of discussion on all of this in the GUIDES Section of the forum if you want to know more about components and choices commonly used in pedals. Especially the Crash Course.