style: change background color to black and adjust video object-fit

Update background color from dark gray to pure black for better contrast
Modify video object-fit to contain to maintain aspect ratio with letterboxing

style.css

@@ -5,7 +5,7 @@
     left: 0;
     width: 100%;
     height: 100%;
-    background-color: #1a1a1a;
+    background-color: #000000;
     display: flex;
     justify-content: center;
     align-items: center;
@@ -57,7 +57,7 @@ html, body {
     width: 100%;
     height: 100%;
     z-index: -1;
-    background-color: #1a1a1a;
+    background-color: #000000;
 }
 
 .bg-video {
@@ -66,7 +66,7 @@ html, body {
     left: 0;
     width: 100%;
     height: 100%;
-    object-fit: cover;
+    object-fit: contain; /* 保持视频比例，留出黑边 */
     opacity: 0;
     transition: opacity 0.2s ease-in-out; /* 交叉淡入淡出动画 */
 }