set background-attachment: fixed for new themes

src/client/styles/agile-admin/inverse/colors/halloween.scss

@@ -65,6 +65,7 @@ $inline-code-bg: #0a121b;
 
 .main-container > #wrapper > #page-wrapper {
   background-image: url("/images/themes/halloween/halloween.png");
+  background-attachment: fixed;
 }
 
 .bg-title {

src/client/styles/agile-admin/inverse/colors/island.scss

@@ -76,6 +76,7 @@ $inline-code-bg: darken($themelight, 3%);
 
 .main-container > #wrapper > #page-wrapper {
   background-image: url("/images/themes/island/island.png");
+  background-attachment: fixed;
 }
 
 /* Tabs */

src/client/styles/agile-admin/inverse/colors/wood.scss

@@ -41,6 +41,7 @@ $inline-code-bg: lighten($subthemecolor, 70%);
 
 .main-container > #wrapper > #page-wrapper {
   background-image: url("/images/themes/wood/wood.png");
+  background-attachment: fixed;
 }
 
 .bg-title {