Docs/CSS: Amend code block rendering.

This commit aims to improve the rendering of code blocks by
reducing the padding, matching the font-size with the rest of the
documentation text and finally, by changing the background colour
back to white from its current gray-ish state which matches the
background colour and making it really hard to spot the code.

Note: The BG colouring issue is due to converting the main BG
to gray whilst missing to change parts that where gray to white, i.e.,
not making the complete switch.

Docker-DCO-1.1-Signed-off-by: O.S. Tezer <ostezer@gmail.com> (github: ostezer)
Merge pull request #5440 from ostezer/docs-improve-code-block-rendering

Docs/CSS: Amend code block rendering.