Remove hardcoded colors from code blocks

[eyegog]

I've encountered some of these issues in my own blog where code blocks do not adhere to the color defined in code-color. Removing the color from poison.css did the trick for me.

before:

After

A live example of the issue is here: https://eyegog.co.uk/posts/a-sad-slow-pppoe-story/

[lukeorth]

Thank you, @eyegog -- I like it!