Logo is not dark mode compatible

[scottgigante-immunai]

Even on the old website, our logo looks bad in dark mode. We should either a) change the grey font color to increase contrast on both white and black backgrounds or b) implement an svg logo so we can set the font colour based on the theme.

[scottgigante-immunai]

TBH I don't think #153 closes this. It's better, sure -- but it would be better still to have a dynamic logo (e.g. through <figure> and @media (prefers-color-scheme: dark)).