base16-github has bad readability

[chawyehsu]

Chars are invisible when using the base16-github scheme.

For comparison:

base16-github	base16-snazzy

Preview of the base16-github scheme, notice that it does not have proper 8 shades of grey:

So the original base16-github-scheme is problematic, I'm considering to fork it and remake the color scheme definition. Would be better to contribute to https://github.com/tinted-theming/schemes/blob/spec-0.11/base16/github.yaml for an update to the scheme, that should follow the official github syntax color scheme:

• https://github.com/primer/github-syntax-theme-generator
• https://github.com/primer/github-syntax-light