diff --git a/.env.development b/.env.development
index 5d818ce..226bc4b 100644
--- a/.env.development
+++ b/.env.development
@@ -6,6 +6,6 @@ NEXT_PUBLIC_USE_MOCK_BACKEND_LOOP_START=false
 NEXT_PUBLIC_EXPORT_STATIC=false
 NEXT_PUBLIC_USE_CGI=false
 # App-Versionsnummer
-NEXT_PUBLIC_APP_VERSION=1.6.869
+NEXT_PUBLIC_APP_VERSION=1.6.870
 NEXT_PUBLIC_CPL_MODE=json   # json (Entwicklungsumgebung) oder jsSimulatedProd (CPL ->CGI-Interface-Simulator) oder production (CPL-> CGI-Interface Platzhalter)
 
diff --git a/.env.production b/.env.production
index 32cfc40..b9091ba 100644
--- a/.env.production
+++ b/.env.production
@@ -5,5 +5,5 @@ NEXT_PUBLIC_CPL_API_PATH=/CPL
 NEXT_PUBLIC_EXPORT_STATIC=true
 NEXT_PUBLIC_USE_CGI=true
 # App-Versionsnummer
-NEXT_PUBLIC_APP_VERSION=1.6.869
+NEXT_PUBLIC_APP_VERSION=1.6.870
 NEXT_PUBLIC_CPL_MODE=production
\ No newline at end of file
diff --git a/CHANGELOG.md b/CHANGELOG.md
index 94f3c0c..df97e2b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,8 @@
+## [1.6.870] – 2025-09-08
+
+- fix: Beim Aufruf der TDR-Detailseite erscheint im Hintergrund auf der KÜ ein Schleifenwiderstand von 0 KOhm. In der Daten Javascriptdatei steht jedoch der richtige Wert.
+
+---
 ## [1.6.869] – 2025-09-08
 
 - fix: Beim Ausführen einer TDR-Messung (Klick auf blauen Button in der TDR-Detailseite) erscheint keine Rückmeldung. Dort müsste ein Hinweis erscheinen “TDR-Messung wird ausgeführt und kann bis zu zwei Minuten dauern”
diff --git a/components/header/Header.tsx b/components/header/Header.tsx
index 4458bcf..57b753a 100644
--- a/components/header/Header.tsx
+++ b/components/header/Header.tsx
@@ -109,19 +109,46 @@ function Header() {
   }, [deviceName, dispatch]);
   //----------------------------------------------------------------
 
-  // Dark/Light Mode Toggle
-  const [isDark, setIsDark] = useState(false);
-  useEffect(() => {
+  // Dark/Light Mode Toggle (persist in localStorage)
+  const [isDark, setIsDark] = useState<boolean>(() => {
     if (typeof window !== "undefined") {
-      const html = document.documentElement;
-      if (isDark) {
-        html.classList.add("dark");
-      } else {
-        html.classList.remove("dark");
+      return document.documentElement.classList.contains("dark");
+    }
+    return false;
+  });
+
+  // Apply + persist when toggled
+  useEffect(() => {
+    if (typeof window === "undefined") return;
+    const html = document.documentElement;
+    if (isDark) {
+      html.classList.add("dark");
+      try {
+        localStorage.setItem("theme", "dark");
+      } catch {
+        /* ignore storage write errors */
+      }
+    } else {
+      html.classList.remove("dark");
+      try {
+        localStorage.setItem("theme", "light");
+      } catch {
+        /* ignore storage write errors */
       }
     }
   }, [isDark]);
 
+  // Sync if another tab changes the theme
+  useEffect(() => {
+    const handler = (e: StorageEvent) => {
+      if (e.key === "theme" && e.newValue) {
+        setIsDark(e.newValue === "dark");
+      }
+    };
+    window.addEventListener("storage", handler);
+    return () => window.removeEventListener("storage", handler);
+  }, []);
+
   return (
     <header className="bg-gray-300 dark:bg-gray-800 flex justify-between items-center w-full h-[13vh] laptop:h-[10vh] relative text-black dark:text-white ">
       <div
diff --git a/package-lock.json b/package-lock.json
index e3499c2..fba1830 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "cpl-v4",
-  "version": "1.6.869",
+  "version": "1.6.870",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "cpl-v4",
-      "version": "1.6.869",
+      "version": "1.6.870",
       "dependencies": {
         "@fontsource/roboto": "^5.1.0",
         "@headlessui/react": "^2.2.4",
diff --git a/package.json b/package.json
index 1d36c77..70dcf05 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "cpl-v4",
-  "version": "1.6.869",
+  "version": "1.6.870",
   "private": true,
   "scripts": {
     "dev": "next dev -p 3000",
diff --git a/pages/_document.tsx b/pages/_document.tsx
index 7098339..4fe3e81 100644
--- a/pages/_document.tsx
+++ b/pages/_document.tsx
@@ -1,15 +1,26 @@
 import { Html, Head, Main, NextScript } from "next/document";
 
+/**
+ * Custom Document to inject an early theme script so the dark-mode preference
+ * (stored in localStorage under key 'theme': 'dark' | 'light') is applied
+ * before the first paint. This verhindert ein "Flash" von Light-Mode beim
+ * Seitenwechsel / Reload in der statisch gebauten Produktion.
+ */
 export default function Document() {
+  const setThemeScript = `(()=>{try{const k='theme';const t=localStorage.getItem(k);const m=window.matchMedia('(prefers-color-scheme: dark)').matches;const useDark = t? t==='dark' : m;const c=document.documentElement.classList;useDark?c.add('dark'):c.remove('dark');}catch(e){/* ignore */}})();`;
   return (
     <Html lang="de">
       <Head>
-        {/* Füge Meta-Tags, CSS-Links und andere Header-Inhalte hier hinzu */}
         <link rel="icon" href="/favicon.png" type="image/png" />
+        {/* Early inline script (no external blocking) */}
+        <script
+          // eslint-disable-next-line react/no-danger
+          dangerouslySetInnerHTML={{ __html: setThemeScript }}
+        />
       </Head>
       <body>
-        <Main /> {/* Hier wird der Seiteninhalt eingebettet */}
-        <NextScript /> {/* Fügt Next.js-Skripte für die Seite hinzu */}
+        <Main />
+        <NextScript />
       </body>
     </Html>
   );