Do not hardcode background for pre code tags

[NicoHood]

2% from fff will result in fafafa
3% will result in f7f7f7

[NicoHood]

I have updated the PR to use $bg-color aka #f8f9fa instead.

1. Both colors should be idential, as one is the block for tagged code (yaml, js, cpp etc) and the other block is for "normal" untagged code.
2. The $bg-color is very close to the current color. It is the same color used in the footer, so it will look nice and is more correct now.