diff --git a/src/App.tsx b/src/App.tsx
index 05e2777..971cadc 100644
--- a/src/App.tsx
+++ b/src/App.tsx
@@ -17,6 +17,7 @@ import DaySchedule from './components/day-schedule';
 import SimulationChart from './components/simulation-chart';
 import Settings from './components/settings';
 import LanguageSelector from './components/language-selector';
+import ThemeSelector from './components/theme-selector';
 import DisclaimerModal from './components/disclaimer-modal';
 import DataManagementModal from './components/data-management-modal';
 import { Button } from './components/ui/button';
@@ -91,6 +92,33 @@ const MedPlanAssistant = () => {
     uiSettings
   } = appState;
 
+  // Apply theme based on user preference or system setting
+  React.useEffect(() => {
+    const theme = uiSettings.theme || 'system';
+    const root = document.documentElement;
+
+    const applyTheme = (isDark: boolean) => {
+      if (isDark) {
+        root.classList.add('dark');
+      } else {
+        root.classList.remove('dark');
+      }
+    };
+
+    if (theme === 'system') {
+      // Detect system preference
+      const mediaQuery = window.matchMedia('(prefers-color-scheme: dark)');
+      applyTheme(mediaQuery.matches);
+
+      // Listen for system theme changes
+      const listener = (e: MediaQueryListEvent) => applyTheme(e.matches);
+      mediaQuery.addEventListener('change', listener);
+      return () => mediaQuery.removeEventListener('change', listener);
+    } else {
+      applyTheme(theme === 'dark');
+    }
+  }, [uiSettings.theme]);
+
   const {
     showDayTimeOnXAxis,
     chartView,
@@ -137,11 +165,18 @@ const MedPlanAssistant = () => {
 
         <div className="max-w-7xl mx-auto">
           <header className="mb-8">
-            <div className="flex justify-between items-start">
+            <div className="flex justify-between items-start gap-4">
             <div>
               <h1 className="text-3xl md:text-4xl font-bold tracking-tight">{t('appTitle')}</h1>
             </div>
-            <LanguageSelector currentLanguage={currentLanguage} onLanguageChange={changeLanguage} t={t} />
+            <div className="flex flex-wrap gap-2 justify-end">
+              <ThemeSelector
+                currentTheme={uiSettings.theme || 'system'}
+                onThemeChange={(theme: 'light' | 'dark' | 'system') => updateUiSetting('theme', theme)}
+                t={t}
+              />
+              <LanguageSelector currentLanguage={currentLanguage} onLanguageChange={changeLanguage} t={t} />
+            </div>
           </div>
           <p className="text-muted-foreground mt-1">{t('appSubtitle')}</p>
         </header>
diff --git a/src/components/language-selector.tsx b/src/components/language-selector.tsx
index 0289fdd..2166564 100644
--- a/src/components/language-selector.tsx
+++ b/src/components/language-selector.tsx
@@ -10,22 +10,18 @@
 
 import React from 'react';
 import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from './ui/select';
-import { Label } from './ui/label';
 
 const LanguageSelector = ({ currentLanguage, onLanguageChange, t }: any) => {
   return (
-    <div className="flex flex-wrap-reverse items-center gap-2">
-      <Label className="text-sm font-medium">{t('languageSelectorLabel')}</Label>
-      <Select value={currentLanguage} onValueChange={onLanguageChange}>
-        <SelectTrigger className="w-32">
-          <SelectValue />
-        </SelectTrigger>
-        <SelectContent>
-          <SelectItem value="en">{t('languageSelectorEN')}</SelectItem>
-          <SelectItem value="de">{t('languageSelectorDE')}</SelectItem>
-        </SelectContent>
-      </Select>
-    </div>
+    <Select value={currentLanguage} onValueChange={onLanguageChange}>
+      <SelectTrigger className="w-32">
+        <SelectValue />
+      </SelectTrigger>
+      <SelectContent>
+        <SelectItem value="en">{t('languageSelectorEN')}</SelectItem>
+        <SelectItem value="de">{t('languageSelectorDE')}</SelectItem>
+      </SelectContent>
+    </Select>
   );
 };
 
diff --git a/src/components/theme-selector.tsx b/src/components/theme-selector.tsx
new file mode 100644
index 0000000..270b5a9
--- /dev/null
+++ b/src/components/theme-selector.tsx
@@ -0,0 +1,29 @@
+/**
+ * Theme Selector Component
+ *
+ * Provides UI for switching between light/dark/system theme modes.
+ * Uses shadcn/ui Select component.
+ *
+ * @author Andreas Weyer
+ * @license MIT
+ */
+
+import React from 'react';
+import { Select, SelectContent, SelectItem, SelectTrigger, SelectValue } from './ui/select';
+
+const ThemeSelector = ({ currentTheme, onThemeChange, t }: any) => {
+  return (
+    <Select value={currentTheme} onValueChange={onThemeChange}>
+      <SelectTrigger className="w-36">
+        <SelectValue />
+      </SelectTrigger>
+      <SelectContent>
+        <SelectItem value="light">{t('themeSelectorLight')}</SelectItem>
+        <SelectItem value="dark">{t('themeSelectorDark')}</SelectItem>
+        <SelectItem value="system">{t('themeSelectorSystem')}</SelectItem>
+      </SelectContent>
+    </Select>
+  );
+};
+
+export default ThemeSelector;
diff --git a/src/constants/defaults.ts b/src/constants/defaults.ts
index 2630134..ef4925e 100644
--- a/src/constants/defaults.ts
+++ b/src/constants/defaults.ts
@@ -99,6 +99,7 @@ export interface UiSettings {
   showTherapeuticRange?: boolean;
   steadyStateDaysEnabled?: boolean;
   stickyChart: boolean;
+  theme?: 'light' | 'dark' | 'system';
 }
 
 export interface AppState {
@@ -171,5 +172,6 @@ export const getDefaultState = (): AppState => ({
     showTherapeuticRange: false,
     steadyStateDaysEnabled: true,
     stickyChart: false,
+    theme: 'system',
   }
 });
diff --git a/src/locales/de.ts b/src/locales/de.ts
index e65bef0..b82ec9d 100644
--- a/src/locales/de.ts
+++ b/src/locales/de.ts
@@ -17,6 +17,11 @@ export const de = {
   languageSelectorEN: "English",
   languageSelectorDE: "Deutsch",
 
+  // Theme selector
+  themeSelectorLight: "☀️ Hell",
+  themeSelectorDark: "🌙 Dunkel",
+  themeSelectorSystem: "💻 System",
+
   // Dose Schedule
   myPlan: "Mein Plan",
   morning: "Morgens",
diff --git a/src/locales/en.ts b/src/locales/en.ts
index 8158a8c..8cd3fb5 100644
--- a/src/locales/en.ts
+++ b/src/locales/en.ts
@@ -17,6 +17,11 @@ export const en = {
   languageSelectorEN: "English",
   languageSelectorDE: "Deutsch",
 
+  // Theme selector
+  themeSelectorLight: "☀️ Light",
+  themeSelectorDark: "🌙 Dark",
+  themeSelectorSystem: "💻 System",
+
   // Dose Schedule
   myPlan: "My Plan",
   morning: "Morning",
diff --git a/src/styles/global.css b/src/styles/global.css
index 73ea027..4a33a11 100644
--- a/src/styles/global.css
+++ b/src/styles/global.css
@@ -31,6 +31,33 @@
     --radius: 0.625rem;
   }
 
+  .dark {
+    --background: 0 0% 10%;
+    --foreground: 0 0% 95%;
+    --card: 0 0% 14%;
+    --card-foreground: 0 0% 95%;
+    --popover: 0 0% 12%;
+    --popover-foreground: 0 0% 95%;
+    --primary: 217 91% 60%;
+    --primary-foreground: 0 0% 100%;
+    --secondary: 220 15% 20%;
+    --secondary-foreground: 0 0% 90%;
+    --muted: 220 10% 18%;
+    --muted-foreground: 0 0% 60%;
+    --accent: 220 10% 18%;
+    --accent-foreground: 0 0% 90%;
+    --destructive: 0 84% 60%;
+    --destructive-foreground: 0 0% 98%;
+    --border: 0 0% 25%;
+    --input: 0 0% 25%;
+    --ring: 0 0% 40%;
+    --chart-1: 12 76% 61%;
+    --chart-2: 173 58% 39%;
+    --chart-3: 197 37% 24%;
+    --chart-4: 43 74% 66%;
+    --chart-5: 27 87% 67%;
+  }
+
   * {
     border-color: hsl(var(--border));
   }