I recommend updating documentation
<h2>How do I set a background color?
Inkscape lets you set a background color in the document properties dialog.
Dont forget set Alpha level, by default is 0. Inkscape doesnt render Alpha but Sozi uses it correctly.
<img> Setting a background color in Inkscape
Unfortunately, the chosen color is only visible in Sozi and Inkscape in images exported from your document. It is ignored by other aplications (web browsers, viewers).
To set a background color directly in svg, you can use the XML editor provided by Inkscape. Select the root <svg:svg> element, add an <svg:style> child element and a text node inside like in the following screenshot: