style(header-dividing): increase background color opacity for enhanced visibility

src/styles/header-dividing.scss

@@ -81,8 +81,7 @@
     z-index: 100;
     --dash-spacing: 6px;
     --dash-width: 2px;
-    background-color: color-mix(in srgb, var(--background) 30%, transparent);
-    opacity: 0.8;
+    background-color: color-mix(in srgb, var(--background) 90%, transparent);
     box-shadow: 0 2px 8px rgba(0, 0, 0, 0.1);
 
     /* 进入动画：透明度由0到1 */