I have made a modification to my Grid + Lightbox css - giving it a transparent background (i.e. no background) so that it will take the BG colour from whatever page or div you set it in.
Have got rid of the annoying little arrow and margin below each thumbnail. All mods now to one stylesheet (disarmed the other - no need for dark/light). Easy to fine tweak borders, spacing, border hovers, title text and background etc. Plain, no-frills grid.
Reduced width to 4 thumbs (546px) wide.