Docs: give the style of the code examples some love

[d-koppenhagen]

I think the code examples could be improved within the docs to be more consistent and beautiful :)

• some code boxes (<pre>) have a black background, some a white one (see example screenshot below)
• font-family monospace is only applied to <pre> but not to <code> within the <pre>
• The style from the code example in "Overview" and "How to use" pages differ from each other

Example / Screenshot: How to use Page

Example / Screenshot: Overview Page