Add dark mode

index.html

@@ -121,7 +121,7 @@
 
                 <div class="control-group">
                     <label>Output Format</label>
-                    <div style="display: flex; gap: 2rem; align-items: center; border: 1px solid black; padding: 1rem; background: #fff;">
+                    <div class="format-selector">
                         <label style="margin:0; display:flex; align-items:center; cursor:pointer;">
                             <input type="radio" name="opt-fmt" value="png" checked style="width:auto; margin-right:0.5rem;">
                             PNG
@@ -133,7 +133,7 @@
                     </div>
                 </div>
             </details>
-            <button id="btn-clear" style="margin-top: 2rem; background: transparent; border: 1px solid black; color: black;">
+            <button id="btn-clear">
                 RESET
             </button>
         </section>