For the love of God, there is no standardized for SVG to set its background color without a rectangle? viewport-fill was just left in limbo?
And as a result, many many many SVG online images that you might want to reuse just rely on white pages and don't add that background rectangle.
Therefore for now we just force white background on our default CSS, which is what most SVGs will work with. Otherwise, you can lose the entire image to our default black background.
Then if someone ever has an SVG that needs another background color, we can add an image attribute to set that color as a local style.