Change the AMP lightbox background

A while ago I mocked up an idea for a mobile friendly site based on a static exhibition, that was closed due to COVID. I just resuscitated the project as a way to test AMP. It all works rather well, in many respects much better than I expected.

I am unsure what the best way to get a white background for the AMP lightbox feature. I tried adding css to the customizer and then created a very simple child theme, now removed. There were a number of places (more than 1) that modifying the background-colour on screen worked. But I think this, which I can see in the inspector:

i-amphtml-lbg-gallery, .i-amphtml-lbg-mask {
background-color: #000 !important;
position: absolute !important;
}

prevents me from changing the bg color. Adding this to the customizer

html[amp] i-amphtml-lbg-gallery, .i-amphtml-lbg-mask {
background-color: whitesmoke;
}

only works with !important, but will not stick.

Can I change the bg color for the AMP lightbox?

Keep up the good work

Many thanks

The page I need help with: [log in to see the link]