added config for (https://js.langchain.com/docs/introduction/) to respect user theme (#8466)

Author: hntrl

docs/core_docs/docusaurus.config.js

@@ -141,6 +141,10 @@ const config = {
   themeConfig:
     /** @type {import('@docusaurus/preset-classic').ThemeConfig} */
     ({
+      colorMode: {
+        disableSwitch: false,
+        respectPrefersColorScheme: true,
+      },
       announcementBar: {
         content:
           '<strong>Our <a href="https://academy.langchain.com/courses/ambient-agents/?utm_medium=internal&utm_source=docs&utm_campaign=q2-2025_ambient-agents_co" target="_blank">Building Ambient Agents with LangGraph</a> course is now available on LangChain Academy!</strong>',