style: use default system theme

docusaurus.config.js

@@ -68,6 +68,10 @@ const config = {
   themeConfig:
     /** @type {import('@acid-info/logos-docusaurus-preset').ThemeConfig} */
     ({
+      colorMode: {
+        disableSwitch: false,
+        respectPrefersColorScheme: true,
+      },
       navbar: {
         items: [
           {