Fix hidden line numbers

The issue is that the `<code>` element needs the `overflow: scroll` (since the code block has a `max-height`). So the line numbers container, which is positioned absolutely using a negative left position is hidden.

This is fixed by changing the way the `<code>` element is positioned: not with a margin and a small size, but with the padding & a larger container.
Further improvements of the coy themes

* Shadows are now visible again!
* Small code cleanup (removed now unnecessary outer `max-height`)
Merge pull request #301 from apfelbox/coy-line-numbers

Fix hidden line numbers in coy theme