Problem with navigation buttons

The reason for the background is the following CSS rule which also applies to the Photoswipe buttons since Photoswipe uses <button> elements for the navigation:

button:not(.toggle), .button, .faux-button, .wp-block-button__link, .wp-block-file .wp-block-file__button, input[type="button"], input[type="reset"], input[type="submit"], .bg-accent, .bg-accent-hover:hover, .bg-accent-hover:focus, :root .has-accent-background-color, .comment-reply-link {
    background-color: 
    #feffff;
}

This is bad style and I consider this a bug in the theme – a CSS rule should never apply globally to all elements without a class (button:not(.toggle)). I don’t know yet how I can create a workaround to fix this. You should ask the creators of Twenty Twenty to fix this CSS rule so it will not affect button elements which are not part of the theme. Maybe you can create a child theme and fix the rules yourself.