Add dark mode

Fix #8

backend/prisma/schema.prisma

@@ -7,6 +7,12 @@ datasource db {
   url      = env("DATABASE_URL")
 }
 
+enum Theme {
+  LIGHT
+  DARK
+  SYSTEM
+}
+
 model User {
   id            Int       @id @default(autoincrement())
   name          String
@@ -14,6 +20,7 @@ model User {
   emailVerified DateTime?
   image         String?
   description   String?   @db.VarChar(512)
+  theme         Theme     @default(SYSTEM)
 
   createdAt DateTime @default(now())
   updatedAt DateTime @updatedAt

backend/src/app/api/auth/theme/route.ts

@@ -0,0 +1,53 @@
+import { NextRequest, NextResponse } from "next/server";
+import z from "zod";
+
+import { auth } from "@/lib/auth";
+import { prisma } from "@/lib/prisma";
+import { RateLimit } from "@/lib/rate-limit";
+
+const themeSchema = z.enum(["LIGHT", "DARK", "SYSTEM"]);
+
+export async function GET(request: NextRequest) {
+	const session = await auth();
+	if (!session || !session.user) return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+
+	try {
+		const user = await prisma.user.findUnique({
+			where: { id: Number(session.user.id) },
+			select: { theme: true },
+		});
+
+		if (!user) return NextResponse.json({ error: "User not found" }, { status: 404 });
+
+		return NextResponse.json({ theme: user.theme });
+	} catch (error) {
+		console.error("Failed to get theme:", error);
+		return NextResponse.json({ error: "Failed to get theme" }, { status: 500 });
+	}
+}
+
+export async function POST(request: NextRequest) {
+	const session = await auth();
+	if (!session || !session.user) return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+
+	const rateLimit = new RateLimit(request, 5);
+	const check = await rateLimit.handle();
+	if (check) return check;
+
+	const { theme } = await request.json();
+
+	const validation = themeSchema.safeParse(theme);
+	if (!validation.success) return rateLimit.sendResponse({ error: "Invalid theme value" }, 400);
+
+	try {
+		await prisma.user.update({
+			where: { id: Number(session.user.id) },
+			data: { theme: validation.data },
+		});
+	} catch (error) {
+		console.error("Failed to update theme:", error);
+		return rateLimit.sendResponse({ error: "Failed to update theme" }, 500);
+	}
+
+	return rateLimit.sendResponse({ success: true });
+}

backend/src/lib/auth.ts

@@ -39,6 +39,8 @@ export const { handlers, signIn, signOut, auth } = NextAuth({
 			if (user) {
 				session.user.id = user.id;
 				session.user.email = user.email;
+				// @ts-expect-error - theme is added to User model
+				session.user.theme = user.theme;
 			}
 			return session;
 		},