style: Update Portal shared background color to #fcfcfc (#8432)

Author: tobiu

resources/scss/src/apps/portal/Viewport.scss

@@ -57,7 +57,7 @@ body {
 
     .portal-shared-background {
         background-attachment: fixed;
-        background-color: var(--sem-color-bg-neutral-default);
+        background-color: #fcfcfc;
         background-image:
             radial-gradient(at 0% 0%, rgba(62, 99, 221, 0.05) 0px, transparent 50%),
             radial-gradient(at 100% 100%, rgba(138, 43, 226, 0.05) 0px, transparent 50%);