Gray background headers

[avivace]

As suggested by @ISSOtm to potentially fix/improve the situation described in #18 , consider gray banded brackground heading/subheadings, as in nocash docs:

This requires (and thus it's blocked by) #18 consistency in the usage of headings (h1-h4)

[ISSOtm]

Not necessarily gray, depending on the color palette we want to have for the page. Variations of green tints could be nice in referring to the console's LCD (though it's difficult to do right).

[ISSOtm]

Closed by #292