feat(desktop): use bg layer 4 for v2 switch (#39085)

packages/ui/src/v2/components/switch-v2.css

@@ -30,7 +30,7 @@
     border: none;
     background:
       linear-gradient(180deg, var(--v2-alpha-light-0) 0%, var(--v2-alpha-light-20) 100%),
-      var(--v2-background-bg-layer-03);
+      var(--v2-background-bg-layer-04);
     box-shadow: var(--v2-elevation-switch-off);
     transition:
       background 90ms ease-out,
@@ -90,7 +90,7 @@
     background:
       linear-gradient(0deg, var(--v2-overlay-simple-overlay-hover), var(--v2-overlay-simple-overlay-hover)),
       linear-gradient(180deg, var(--v2-alpha-light-0) 0%, var(--v2-alpha-light-20) 100%),
-      var(--v2-background-bg-layer-03);
+      var(--v2-background-bg-layer-04);
   }
 
   &:hover:not([data-disabled], [data-readonly]) [data-slot="switch-thumb"] {