I learned this trick in a card-making class I took last year.
If you want a black border around your main 12x12 background but don't have a piece of 12x12 in black, take a Zig writer or black marker and draw one. You will need to draw straight lines with a ruler, but it's a simple procedure, and it looks as though you have an entire sheet of 12x12 black under your background paper.

I only ever tried it with black because it's my favorite background color, and I didn't have a Zig writer in any other color, but I'm sure it would work in colors other than black.