Dark mode toggling switch will not show when 'repo' is not defined in themeConfig

[MetalOxideSemi]

When the themeConfig property in config.js is defined as

module.exports = {
    theme: "vt",
    themeConfig: {
        enableDarkMode: true,
        repo: "https://google.com",
    },
};

the dark mode toggling switch is shown, even though the repo url is not a real repository. But when it is defined as

module.exports = {
    theme: "vt",
    themeConfig: {
        enableDarkMode: true,
    },
};

The dark mode toggling switch will be simply dissappeared.

Does the repo url definition have anything to do with the dark mode switch? I thought they were two unrelated features...