Improve contrast of code highlighting

[fonsp]

Our code does not always have good readable contrast.

For example, strings like md""" have low contrast, Chrome also points this out:

@savq Offered to investigate this (which scoring function should be used for contrast?) and maybe make a new color theme for us! It might be fun to release it together with https://github.com/JuliaPluto/lezer-julia/pull/7 so that things "look new"