feat: Add github-default-dark theme

[wiidotmom]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) Adds github-default-dark theme based off of GitHub's Default dark theme

What is the current behavior? (You can also link to an open issue here) The existing github-dark theme doesn't match the Default dark GitHub site theme

What is the new behavior (if this is a feature change)? n/a

Additional info: Didn't include an author section as I didn't really come up with the theme, only implemented it for fig (the existing github-dark theme didn't have one either). Let me know if an author section is needed and I can update the PR.