refactor: unify base styles backdrop property name and default value (#10080)

Co-authored-by: Jouni Koivuviita <jouni@vaadin.com>

Author: web-padawan

packages/app-layout/src/styles/vaadin-app-layout-base-styles.js

@@ -127,7 +127,7 @@ export const appLayoutStyles = css`
   }
 
   [part='backdrop'] {
-    background: var(--vaadin-overlay-backdrop-background, rgba(0, 0, 0, 0.5));
+    background: var(--vaadin-overlay-backdrop-background, rgba(0, 0, 0, 0.2));
   }
 
   :host(:not([drawer-opened])) [part='backdrop'] {