Comment's darkTheme doesn't work.

In siteMetadata.js comments.giscusConfig.darkTheme specifying 'dark' doesn't seem to work. The comments theme stays as the original theme value.

My config:

// ...
comments: {
    // ...
    giscusConfig: {
        theme: 'light', 
        themeDark: 'dark', 
        // ...
    },
    // ...
}
// ...

Is there something I'm missing?