Formulae with the GitHub dark theme

[aromanro]

First, thank you for the tutorials, they proved very useful to me, I extended my https://github.com/aromanro/HartreeFock project starting from your tutorials (unfortunately I found them quite late, I already had HF working).

I opened this just to let you know that the formulae seem to have a transparent background and they are almost invisible with the GitHub dark theme.

Thank you again for making this available!