Fix the frame that is always on top on the black side

[jrocha-io]

Good day! In both code that is in this repository and in the codepen, one of the frames on the side of the black always ends on top. When you turn on the borders, so does a group of frames on the white side (the frames whose contents are the column indicating letters). To fix this problem, we should simply add the line

z-index: -1;

at line 102 of the styles.css file, as a property of the selectors

#board: after, #frame: after, #frame: before.

[jrocha-io]

Sorry, i forgot: for the group of frames on the white side, we need to add 'z-index: 3;' in .piece (line 225).