Fix hidden line numbers in coy theme

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 & and using the complete size of the `<pre>` tag. This way the line numbers container is in the visible area of the `<code>`-element.

Fixes #188
Note: this also slightly changes the way the theme looks (see the linked issue for screenshots).