Lock navbar and add border/shadow styling

Author: ankitkr104

assets/css/custom.scss

@@ -74,6 +74,15 @@ blockquote {
   padding-top: 5px;
 }
 
+.main-nav {
+  position: sticky;
+  top: 0;
+  z-index: 1000;
+  background: $bg-white;
+  border-bottom: 1px solid $border-color;
+  box-shadow: 0 6px 18px rgba(27, 31, 35, 0.08);
+}
+
 .breadcrumb {
   background: #F3F3F3;