[modern] Drop the background gradient

Move content border rules to layout (will be removed).

Bug: T251583
Change-Id: I28b7a3fdd3f89a0aef93e64c1ae12f39ae430e66

resources/skins.vector.styles/index.less

@@ -5,8 +5,6 @@
 
 @media screen {
 	@import 'layout.less';
-	// FIXME: Will be removed with new header.
-	@import 'legacy/background-gradient.less';
 	@import 'common/content.less';
 	@import 'common/typography.less';
 

resources/skins.vector.styles/layout.less

@@ -43,6 +43,10 @@ body {
 }
 
 .mw-body {
+	// Will be removed when we limit content width (T246420).
+	/* Border on top, left, and bottom side */
+	border: @border-width-base @border-style-base @border-color-content;
+	border-right-width: 0;
 	/* Merge the border with tabs' one (in their background image) */
 	margin-top: -@border-width-base;