Set media background to quarto body background

[cderv]

This will allow transparent images to be seen on the media background

Closes #9047

format: 
  html:
    backgroundcolor: "red"

@dragonstyle do you think this is not a good idea ? I believe non-transparent images won't be affected by a set background on the div.

[cscheid]

We do have to be careful here; if someone uses a background color that has partial alpha, I think this rule won't work (it will "double-paint" the background).

[cderv]

If someone does this in the image aim to be in the lightbox, it will also apply on the image itself in the doc, won't it ? We do set background for the document I believe... I'll try it to experiment.

Or do you mean someone who would do that specifically for the image when inside the lightbox modal ?

Anyway, thanks for the feedback - I did not merge because I felt something was not enough.

[cscheid]

I meant that we should test what happens if someone sets quarto body background to be rgba(255,0,0,0.5) (the 0.5 being the important part). If I understand your patch correctly, you'd be adding an additional rgba(255,0,0,0.5) to the image and that would mean that visually the lightbox image would end up with rgba(255,0,0,0.75) because of the semi-transparent composition.

[cderv]

Thanks for the precision - I understand now. And yes indeed it would lead to

FWIW we already have this issue for the description that we set using background color too.

My patch was inspired by this treatment

quarto-cli/src/resources/formats/html/glightbox/lightbox.css

Lines 1 to 6 in e34c7a0

	body:not(.glightbox-mobile) div.gslide div.gslide-description,
	body:not(.glightbox-mobile) div.gslide-description .gslide-title,
	body:not(.glightbox-mobile) div.gslide-description .gslide-desc {
	color: var(--quarto-body-color);
	background-color: var(--quarto-body-bg);
	}

We apply to the .gslide-description but also to .gslide-title and .gslide-desc - so we have even more layer of opacity

This is run with

format: html
backgroundcolor: rgba(255,0,0,0.5)

[cscheid]

Maybe what we should do then is just add some documentation that recommends opacity either zero or one.