Update theme setup

src/lib/ThemeContext.svelte

@@ -0,0 +1,13 @@
+<svelte:head>
+    <script>
+        if (document) {
+            const defaultTheme = 'dark';
+            const itemTheme = localStorage.getItem("theme") ?? defaultTheme;
+
+            localStorage.setItem("theme", itemTheme);
+            document.documentElement.setAttribute("data-theme", itemTheme);
+        }
+    </script>
+</svelte:head>
+
+<slot></slot>