Tags: °Opera

To do this in Opera, you need to modify Styles/image.css and change the background-color attribute to hsl(0, 0%, 0%), or #000000.
Upgrades often replace this image.css, so you'll need to re-apply the change after updating to a new version.  You can keep a "good" image.css file in the styles folder ready to copy back into place to make things easier.

Below is an Image.css with black backgrounds applied and the annoying automatic scaling "feature" disabled:

@charset "utf-8";
/* Stylesheet for the Opera image viewer */
/* Copyright 2011 Opera Software */
html {
    background-color: hsl(0,0%,0%);
    display: table;
    width: 100%;
    height: 100%;
}
img {
    display: block;
    margin: auto;
}
body {
    margin: 0;
    padding: 8px;
    display: table-cell;
    vertical-align: middle;
}

@media projection, tv, handheld {
html {background-color: black;}
body {background-color: hsl(0,0%,39%);}
img {background-color: hsl(0,0%,98%);}
}

/* Disable auto resizing of large images code starts here */
*, *::before, *::after {
    position: static !important;
    height: inherit !important;
    width: inherit !important;
    cursor: default !important;
    padding: 0px !important;
    margin-left:auto;
    margin-right:auto;
}
/* Disable auto resizing of large images code ends here */