Fix dark theme's code block background, line number colors (#1124)

When customizing `$code-background-color` in `dark.scss`, the result is a multi-color background. (see https://github.com/just-the-docs/just-the-docs/issues/1121#issuecomment-1374976843)

This makes OneDarkJekyll code block colors the same as the specified `$code-background-color`, and uses the `$default-body-color` as the line number text color, which can otherwise be invisible due to the default being black and is hard to see on a very dark code block background.