N4 lets the background from any table cell or <div> inherit into any
tables inside it. This stuffs things up a treat. You can override the
background image inheritance in HTML using <table background=""> or CSS
'background-image: none;', but you cannot override the background colour
inheritance - 'background-color: transparent;' does not work.

avoid setting background colours on elements which contain tables (use
an image instead if necessary). Also, N4 takes a 'background: url(...);'
rule as specifying the relevant background-image *and* setting the
background-color to black, so even if you don't think you're setting a
background colour, you are if you use this shortcut. Instead use
'background-image' and the other background- properties.

Or withhold styles from N4 and ignore the problem.