css: Move out --box-shadow-overlay out of dark_theme.css.

Fixes #35881.

web/styles/app_components.css

@@ -1118,6 +1118,12 @@ input.settings_text_input {
     }
 }
 
+div.overlay .flex.overlay-content > .overlay-container,
+#settings_page,
+.informational-overlays .overlay-content {
+    box-shadow: var(--box-shadow-overlay);
+}
+
 .delete-selected-drafts-button-container {
     display: flex;
 }

web/styles/app_variables.css

@@ -404,6 +404,9 @@
     */
     --message-area-max-width: 57.4125em;
 
+    /* Box shadow for overlays across the web app */
+    --box-shadow-overlay: none;
+
     /* Shared sidebar typography and effects values. */
     --font-weight-sidebar-heading: 600;
     --font-weight-sidebar-action-heading: 370;
@@ -3010,6 +3013,9 @@
        the unread marker to the left. */
     --unread-marker-left: 0;
 
+    /* Box shadow for overlays across the web app */
+    --box-shadow-overlay: 0 0 30px hsl(212deg 32% 7%);
+
     /* Colors used across the app */
     --color-recipient-bar-controls-spinner: hsl(0deg 0% 100%);
     --box-shadow-unread-counter-normal: none;

web/styles/dark_theme.css

@@ -107,12 +107,6 @@
         border-color: hsl(0deg 0% 0% / 90%);
     }
 
-    & div.overlay .flex.overlay-content > .overlay-container,
-    #settings_page,
-    .informational-overlays .overlay-content {
-        box-shadow: 0 0 30px hsl(212deg 32% 7%);
-    }
-
     .popover hr,
     hr {
         opacity: 0.2;