Fix regression with sticky toolbar border (#6980)

The merge of a more background color friendly UI introduced a regression where the bottom border of the toolbar was gone, to avoid stacking the transparent opacities.

This PR reintroduces the solid color, and the bottom border. It's still friendly to colored backgrounds.

editor/components/block-list/style.scss

@@ -717,11 +717,7 @@
 	// use opacity to work in various editor styles
 	background-clip: padding-box;
 	box-sizing: padding-box;
-	border: 1px solid $dark-opacity-light-500;
-
-	.is-dark-theme & {
-		border-color: $light-opacity-light-500;
-	}
+	border: 1px solid $light-gray-500;
 
 	// this prevents floats from messing up the position
 	position: absolute;
@@ -737,10 +733,6 @@
 	}
 }
 
-.editor-block-contextual-toolbar .editor-block-toolbar {
-	border-bottom: none;
-}
-
 .editor-block-list__breadcrumb .components-toolbar {
 	padding: 0px 12px;
 	line-height: $block-toolbar-height - 1px;