Comment 1 for bug 1800536

Workaround (as Sozi app does) is to read background color from svg pagecolor and style it externaly:
<style xmlns="http://www.w3.org/1999/xhtml">svg {background: rgba(56, 128, 131, 1); }</style>

And next problem - by default it also didnt work because you didnt notice opacity of background set to 0. Opacity value doesnt affect background color in Inkscape, in other apps is it rendered correctly and you dont notice it by first time:

Opacity 0 (default):

Opacity 255:

http://sozi.baierouge.fr/community/d/65-change-background-of-svg/7