Update theme handling and add theme selection dropdown in Navbar.svelte

src/lib/components/Navbar.svelte

@@ -97,5 +97,18 @@
       <UserAvatar {user} />
     {/if}
     <button class="btn btn-neutral ml-4" on:click={showModal}>About</button>
+    <div class="dropdown dropdown-bottom dropdown-end">
+      <div tabindex="0" role="button" class="btn m-1 ml-4">Themes</div>
+      <!-- svelte-ignore a11y-no-noninteractive-tabindex -->
+      <ul
+        tabindex="0"
+        class="dropdown-content z-[1] menu p-2 shadow bg-base-100 rounded-box w-52"
+      >
+        <form method="POST">
+          <li><button formaction="/?/setTheme&theme=light">Light</button></li>
+          <li><button formaction="/?/setTheme&theme=dark">Dark</button></li>
+        </form>
+      </ul>
+    </div>
   </div>
 </div>

src/lib/components/UserAvatar.svelte

@@ -37,8 +37,8 @@
     <li><a>Profile</a></li>
     <li><button on:click={navToLog}>My Log</button></li>
     <li><button on:click={navToSettings}>Settings</button></li>
-    <form method="post" action="/" use:enhance>
-      <li><button>Logout</button></li>
+    <form method="post">
+      <li><button formaction="/?/logout">Logout</button></li>
     </form>
   </ul>
 </div>