Lightbox (single Image): Space arround Image?

How to control the design of the lightbox (single image, no gallery)?
When I click on the image, this image opens in the lightbox format-filling. But I would like to have a certain space around the image of, say 10%.

I tried editing the css of the photoswipe elements and achieve the desired effect with:

.pswp img {
    scale: 0.9;
}

But this results in an ugly, stuttering effect with the animation of the lightbox.

Is there a better way?
Thanks for any help!

-kai