support theme based background color

[jakoubektom]

I would like to be able set different background color for light and dark theme, similar to what is supported in https://github.com/gabrielcsapo/docusaurus-plugin-image-zoom

imageZoom: {
    background: {
        dark: darkCodeTheme.plain.backgroundColor,
        light: lightCodeTheme.plain.backgroundColor,
    }
}