This is a pain point as of SVG 1.1...
Examples at svg/background.html which answers from stackoverflow.com/questions/11293026/default-background-color-of-svg-root-element/11293812:
- svg/background-rect.svg
- svg/background-viewport-fill.svg: was part of SVG 1.2, but that whole standard got dropped. Not implemented neither in Chromium 85 nor Firefox 93 as of 2021.
This pain reflects directly on Inkscape: set SVG background color in Inkscape.