validate-theme-config-color-mode-default-mode

Require themeConfig.colorMode.defaultMode to be present and to use canonical "light" or "dark" values when colorMode config is authored statically.

Targeted pattern scope

This rule targets docusaurus.config.* files and inspects themeConfig.colorMode.defaultMode.

What this rule reports

This rule reports when defaultMode is:

• missing from a statically authored colorMode object,
• a statically invalid value, or
• a valid value with non-canonical casing/whitespace.

Why this rule exists

Color mode defaults affect first-load UX and should be explicit and predictable in authored config.

❌ Incorrect

export default {
    themeConfig: {
        colorMode: {},
    },
};

export default {
    themeConfig: {
        colorMode: { defaultMode: "Dark" },
    },
};

✅ Correct

export default {
    themeConfig: {
        colorMode: { defaultMode: "dark" },
    },
};

Behavior and migration notes

This rule autofixes valid-but-noncanonical static string values such as "Dark" to "dark".

When defaultMode is missing or statically invalid, it reports and provides suggestions to set it to "light" or "dark".

When not to use it

Do not use this rule if your project intentionally leaves defaultMode implicit or resolves it outside static config.

Rule catalog ID: R078

Further reading

• Docusaurus color mode configuration