Thank you so much for that. It solves my problem of showing the edges of the prints and drawings on the website I am making, without having to put artificial borders on each image. The area surrounding each print in its modal window can now match the background of the main page. With the default white background it was not possible to see the actual shape of each print. I am not at all au-fait with code and it always astonishes me what a series of letters and numbers can do.
And by the way I am very much enjoying the freedom and vast expanse of possibilities which the Uikit framework allows.
Here it shows the effect in practice in the, so far, unfinished website: