fix: improve theme consistency and layout

- Added 8px gap to button groups for better spacing
- Fixed header/nav background overlay issue (consolidated styling)
- Added gray background to main container (metal-bg-surface)
- Fixed heading section (removed margin, border-radius, transparent bg)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

asciinema/theme/custom.css

@@ -56,10 +56,9 @@ h1, h2, .h1, .h2 {
 }
 
 .heading {
-  background-color: var(--metal-bg-elevated) !important;
+  background-color: transparent !important;
-  padding: 24px !important;
+  padding: 0 0 24px 0 !important;
-  margin: -16px -16px 24px -16px !important;
+  margin: 0 !important;
-  border-radius: 8px 8px 0 0 !important;
 }
 
 .heading h2 {
@@ -70,14 +69,13 @@ h1, h2, .h1, .h2 {
 
 /* Header & Navigation */
 header,
-.header,
+.header {
-nav,
-.navbar {
   background-color: var(--metal-bg-elevated) !important;
   border-bottom: 2px solid var(--pivoine-rose) !important;
 }
 
-/* Override Bootstrap navbar-dark and bg-dark */
+nav,
+.navbar,
 .navbar-dark,
 .bg-dark {
   background-color: var(--metal-bg-elevated) !important;
@@ -136,6 +134,11 @@ input[type="submit"]:active {
   box-shadow: 0 2px 4px rgba(0, 0, 0, 0.2);
 }
 
+/* Button groups */
+.btn-group {
+  gap: 8px !important;
+}
+
 /* Filter buttons (btn-light in btn-group) */
 .btn-group .btn,
 .btn-light {
@@ -198,6 +201,13 @@ article {
   padding-right: 15px !important;
 }
 
+/* Main container background */
+main .container {
+  background-color: var(--metal-bg-surface) !important;
+  padding: 24px !important;
+  border-radius: 8px !important;
+}
+
 /* Forms & Inputs */
 input[type="text"],
 input[type="email"],