Make themes & code highlight colours aware of each other

[dbkr]

Themes can have light or dark background and our code highlighting library, highlight.js, has light and dark themes, but we just include the github light theme, overriding some of the colours. This leads to bugs like #19476

We should make them aware of each other somehow, eg. having a light/dark discriminator for themes so we can pick an appropriate highlight.js theme, or perhaps just don't use highlight.js's themes at all and define the colours in our own themes.

There might also be other areas of the app where we need to make this distinction.

[dbkr]

Never mind - the include of the github light theme was presumably a leftover from something and was the cause of that bug. We include light/dark highlight.js themes from the light/dark themes respectively.