diff --git a/src/components/ThemeSelector.tsx b/src/components/ThemeSelector.tsx
index 0ea96ab..a9c8d37 100644
--- a/src/components/ThemeSelector.tsx
+++ b/src/components/ThemeSelector.tsx
@@ -1,6 +1,23 @@
 import { useEffect, useState } from "react";
 
-type Theme = "dark" | "light";
+type Theme = "dark" | "light" | "automatic";
+
+const DiagonalSplitPreview = () => {
+  return (
+    <svg
+      width="64"
+      height="40"
+      viewBox="0 0 64 40"
+      className="w-full h-full"
+      preserveAspectRatio="none"
+    >
+      {/* Upper triangle (light) */}
+      <polygon points="0,0 64,0 64,40" fill="#ebebeb" />
+      {/* Lower triangle (dark) */}
+      <polygon points="0,0 0,40 64,40" fill="#2d2d2d" />
+    </svg>
+  );
+};
 
 const ThemeOption = ({
   value,
@@ -12,7 +29,7 @@ const ThemeOption = ({
 }: {
   value: Theme;
   label: string;
-  previewColor: string;
+  previewColor: string | "diagonalSplit";
   theme: Theme;
   setTheme: (theme: Theme) => void;
   isLast?: boolean;
@@ -40,10 +57,16 @@ const ThemeOption = ({
     >
       <div className="flex items-center gap-6">
         {/* Preview Box */}
-        <div
-          className="w-16 h-10 border border-switch-line-sep shadow-sm"
-          style={{ backgroundColor: previewColor }}
-        />
+        <div className="w-16 h-10 border border-switch-line-sep overflow-hidden">
+          {previewColor === "diagonalSplit" ? (
+            <DiagonalSplitPreview />
+          ) : (
+            <div
+              className="w-full h-full"
+              style={{ backgroundColor: previewColor }}
+            />
+          )}
+        </div>
 
         {/* Label */}
         <span className="text-xl font-normal text-switch-text">{label}</span>
@@ -68,19 +91,66 @@ const ThemeOption = ({
   );
 };
 
+function getSystemTheme(): "dark" | "light" {
+  if (typeof window === "undefined") return "dark";
+  return window.matchMedia("(prefers-color-scheme: dark)").matches
+    ? "dark"
+    : "light";
+}
+
+function getEffectiveTheme(theme: Theme): "dark" | "light" {
+  if (theme === "automatic") {
+    return getSystemTheme();
+  }
+  return theme;
+}
+
 export function ThemeSelector() {
   const [theme, setTheme] = useState<Theme>(() => {
     const saved = localStorage.getItem("theme") as Theme | null;
-    return saved || "dark";
+    // Default to "automatic" if no saved preference
+    return saved || "automatic";
   });
 
   useEffect(() => {
-    document.documentElement.setAttribute("data-theme", theme);
+    const effectiveTheme = getEffectiveTheme(theme);
+    document.documentElement.setAttribute("data-theme", effectiveTheme);
     localStorage.setItem("theme", theme);
   }, [theme]);
 
+  // Listen to system theme changes when "automatic" is selected
+  useEffect(() => {
+    if (theme !== "automatic") return;
+
+    const mediaQuery = window.matchMedia("(prefers-color-scheme: dark)");
+    const handleChange = () => {
+      if (theme === "automatic") {
+        const newSystemTheme = getSystemTheme();
+        document.documentElement.setAttribute("data-theme", newSystemTheme);
+      }
+    };
+
+    // Modern browsers
+    if (mediaQuery.addEventListener) {
+      mediaQuery.addEventListener("change", handleChange);
+      return () => mediaQuery.removeEventListener("change", handleChange);
+    }
+    // Fallback for older browsers
+    else if (mediaQuery.addListener) {
+      mediaQuery.addListener(handleChange);
+      return () => mediaQuery.removeListener(handleChange);
+    }
+  }, [theme]);
+
   return (
     <div className="space-y-0" role="radiogroup" aria-label="Theme Selection">
+      <ThemeOption
+        value="automatic"
+        label="Automatic"
+        previewColor="diagonalSplit"
+        theme={theme}
+        setTheme={setTheme}
+      />
       <ThemeOption
         value="light"
         label="Basic White"
diff --git a/src/index.css b/src/index.css
index f3afd8c..2e25a37 100644
--- a/src/index.css
+++ b/src/index.css
@@ -80,9 +80,6 @@
     min-height: 100vh;
     background-color: var(--color-switch-selected-bg);
     color: var(--switch-text);
-    transition:
-      background-color 0.3s,
-      color 0.3s;
   }
 
   /* Custom Scrollbar */
diff --git a/src/main.tsx b/src/main.tsx
index eff7ccc..a5d3276 100644
--- a/src/main.tsx
+++ b/src/main.tsx
@@ -3,6 +3,24 @@ import { createRoot } from "react-dom/client";
 import "./index.css";
 import App from "./App.tsx";
 
+// Set initial theme before React renders to avoid flash
+function initializeTheme() {
+  const saved = localStorage.getItem("theme");
+  let effectiveTheme: "dark" | "light";
+
+  if (saved === "light" || saved === "dark") {
+    effectiveTheme = saved;
+  } else {
+    // Default to automatic (system preference)
+    const prefersDark = window.matchMedia("(prefers-color-scheme: dark)").matches;
+    effectiveTheme = prefersDark ? "dark" : "light";
+  }
+
+  document.documentElement.setAttribute("data-theme", effectiveTheme);
+}
+
+initializeTheme();
+
 createRoot(document.getElementById("root")!).render(
   <StrictMode>
     <App />