添加深色模式切换功能，优化样式以支持深色主题

src/style.css

@@ -7,6 +7,9 @@
   color: rgba(255, 255, 255, 0.87);
   background-color: #242424;
 
+  --background-color: #242424;
+  --text-color: #3d3d3d;
+
   font-synthesis: none;
   text-rendering: optimizeLegibility;
   -webkit-font-smoothing: antialiased;
@@ -18,10 +21,16 @@ a {
   color: #646cff;
   text-decoration: inherit;
 }
+
 a:hover {
   color: #535bf2;
 }
 
+.dark {
+  --background-color: #242424;
+  --text-color: rgb(228, 228, 228);
+}
+
 body {
   margin: 0;
   display: flex;
@@ -42,7 +51,8 @@ button {
   font-size: 1em;
   font-weight: 500;
   font-family: inherit;
-  background-color: #1a1a1a;
+  background-color: var(--background-color);
+  color: var(--text-color);
   cursor: pointer;
   transition: border-color 0.25s;
 }
@@ -77,3 +87,7 @@ button:focus-visible {
     background-color: #f9f9f9;
   }
 }
+
+p,a,h1,h2,h3,h4,h5,h6 {
+  color: var(--text-color);
+}