Add new theme #knochenmark v1.0

Hello again, this is a second theme I did, more for fun ;). The theme shows the "bones" of markdown, as the name suggests.
I created the branch out of the branch for the first theme, I have no idea if that's how it should be done, because I see the files for the first theme again in the changes below but I guess if you merge the first pull request first, the remaining differences should only be the files of the second theme (?).