fix(overlays): use the fallback background variable for theming

fixes #18658

Author: brandyscarney

core/src/themes/ionic.theme.default.md.scss

@@ -10,7 +10,7 @@
 $backdrop-md-color:                         var(--ion-backdrop-color, #000) !default;
 $border-md-color:                           var(--ion-border-color, var(--ion-color-step-150, #c1c4cd)) !default;
 $box-shadow-md-color:                       var(--ion-box-shadow-color, #000) !default;
-$overlay-md-background-color:               var(--ion-overlay-background-color, #fff) !default;
+$overlay-md-background-color:               var(--ion-overlay-background-color, var(--ion-background-color, #fff)) !default;
 
 // Material Design Tabs & Tab bar
 // --------------------------------------------------