[book] Code blocks in light modes are poorly readable

[sgued]

Hi! Thanks for all your work on helix, it's great!

Code blocks in the book are barely readable in the light themes of the book, because some color variables are taken from the dark themes:

This is true both for inline code blocks and for normal code blocks.

For reference, this is what it looks like without any custom CSS:

It is overall much more readable in a light mode context.

I also noticed that the default theme is a dark one. IMO this is a mistake and the default theme should be a light theme. mdbook is smart enough to read use the browser's setting and switch to preferred-dark-theme when appropriate.

[rgoulter]

The Configuration page has a particularly hard to read part: