Refine multiselect. (#19121)

* Refine multiselect.

This restores the native selection in addition to the borders.

Additionally, it changes the cross-block selection border to also have a white outline.

* Refine colors, add dark theme exception.

packages/base-styles/_colors.scss

@@ -83,6 +83,7 @@ $blue-medium-200: #bfe7f3;
 $blue-medium-100: #e5f5fa;
 $blue-medium-highlight: #b3e7fe;
 $blue-medium-focus: #007cba;
+$blue-medium-focus-dark: #fff;
 
 // Alert colors.
 $alert-yellow: #f0b849;

packages/block-editor/src/components/block-list/style.scss

@@ -182,11 +182,16 @@
 	.block-editor-block-list__block.is-multi-selected {
 		// Show selection borders around every non-nested block's actual footprint.
 		> .block-editor-block-list__block-edit > [data-block] {
-			box-shadow: 0 0 0 2px #057cba;
+			box-shadow: 0 0 0 2px $blue-medium-focus;
 			border-radius: 1px;
 
 			// Windows High Contrast mode will show this outline.
 			outline: 2px solid transparent;
+
+			// Show a lighter blue for dark themes.
+			.is-dark-theme & {
+				box-shadow: 0 0 0 2px $blue-medium-focus-dark;
+			}
 		}
 		.block-editor-block-toolbar {
 			left: $border-width + $border-width + $block-padding;
@@ -195,9 +200,11 @@
 			margin-left: $block-padding;
 		}
 
-		// Hide the native selection marker once we select multiple blocks.
-		::selection {
-			background: transparent;
+		// Provide exceptions for placeholders.
+		.components-placeholder {
+			::selection {
+				background: transparent;
+			}
 		}
 	}