style: add subtle gradient backgrounds for dark and light themes

Dark: navy → indigo → purple gradient
Light: white → indigo → pink gradient

Author: lambdalisue

static/style.css

@@ -24,6 +24,13 @@ html {
   --hero-gradient: rgba(99, 102, 241, 0.15);
   --link-color: var(--accent-light);
   --header-bg: rgba(15, 23, 42, 0.1);
+  --bg-gradient: linear-gradient(
+    135deg,
+    #0f172a 0%,
+    #1e1b4b 35%,
+    #2e1065 65%,
+    #0f172a 100%
+  );
 }
 
 /* Light theme */
@@ -38,6 +45,13 @@ html {
   --hero-gradient: rgba(99, 102, 241, 0.08);
   --link-color: var(--accent-dark);
   --header-bg: rgba(248, 250, 252, 0.1);
+  --bg-gradient: linear-gradient(
+    135deg,
+    #f8fafc 0%,
+    #e0e7ff 35%,
+    #fae8ff 65%,
+    #f8fafc 100%
+  );
 }
 
 /* ========================================
@@ -49,12 +63,13 @@ html {
 
 body {
   font-family: "Inter", -apple-system, BlinkMacSystemFont, sans-serif;
-  background: var(--bg-primary);
+  background: var(--bg-gradient);
+  background-attachment: fixed;
   color: var(--text-primary);
   margin: 0;
   min-height: 100vh;
   line-height: 1.6;
-  transition: background-color 0.3s, color 0.3s;
+  transition: background 0.3s, color 0.3s;
 }
 
 a {