Dark mode GitHub README logo breaks accessbility guidelines

[agriyakhetarpal]

See: https://webaim.org/resources/contrastchecker/?fcolor=636363&bcolor=0E1117

Some possible ways of resolving this are:

• use a white background PNG image instead of a transparent background one
• Add a separate logo for dark mode for GitHub with white text (similar to the one I added for the PyBaMM home and the PyBaMM documentation websites)

The first one sounds better to me – since there won't be a way to control the logo's display in light vs. dark mode, and modifying the logo to have white text would make it invisible for users scrolling on GitHub in light mode.