It appears that your text file is encoded as text/plain whereas the browser wants a text/css format. It appears your stylesheet is not loading at all.
I found a post on StackOverflow that might help:
Found something really cool. You get the raw link as: http://raw.github.com/...
Simply fetch the files from rawgit.com (or cdn.rawgit.com ) instead of raw.github.com and DONE! UPDATE:
You can also use raw.githack.com if you want to serves raw files directly from Bitbucket or GitLab
P.S: For styling buttons i just use an <a> element with display: block;, because it’s more cross-compatible (but if this works for you that’s fine; just a personal preference!)