Fix default code text color

[jjspace]

Fixes #131

VSCode seems to now include a default style for functions and params that was not overridden in the extension and resulted in colors that are hard to read, specifically in light mode.

The problem:

.hljs-subst, .hljs-function, .hljs-title, .hljs-params, .hljs-formula {
    color: rgb(220, 220, 220);
}

This PR updates the function and params colors to use the default fg from the github styles to match closer to how it appears on Github (highlight.js doesn't perfectly match)

I also noticed the .markdown-body .hljs doesn't seem to exist anymore in the MD preview so it wasn't correctly styling the normal text like () and {} so I expanded that selector.

I also updated generate-github-markdown-css because Github made some changes that made it no longer parse correctly. See sindresorhus/generate-github-markdown-css#27 and sindresorhus/generate-github-markdown-css#28

Testing

Add this cpp code block to a test markdown and preview it in all themes and ensure it's readable

#include <cstdin>

int main () {
  int x = 0;
  std::thread t1(inc, std::ref(x), 1);
  std::thread t2(inc, std::ref(x), 2);
  std::thread t3(inc, std::ref(x), 3);
  std::thread t4(inc, std::ref(x), 4);
}

[mjbvz]

Thanks!