Adapt SVGs to book theme

[ericthelemur]

This issue has been discussed in #74, and I've described my approach there. The idea is to #include the SVGs (avoiding Markdown formatting) and use existing CSS variables to match the theme.

This will fall back to the original black and white if the CSS variables are unavailable.

Example in context:

[wasd96040501]

@ericthelemur Any progress on this?

[ericthelemur]

I haven't looked at this for a while now, but I think it's still correct and complete. This solution has continued to worked well on my own mdbook. The only downside is having to inline svgs.

I'll rebase it to current and check it is still valid