Morden: Fix responsive logic in header

morden/sass/_extra-child-theme.scss

@@ -68,8 +68,13 @@ body {
 	position: relative;
 
 	.site-header-wrap {
-
 		@extend %responsive-alignwide;
+		margin-left: auto;
+		margin-right: auto;
+
+		@include media(mobile-only) {
+			width: calc(100% - #{2 * map-deep-get($config-global, "spacing", "unit")});
+		}
 
 		& > * {
 			max-width: 100%;

morden/style-rtl.css

@@ -3491,6 +3491,17 @@ hr.wp-block-separator.is-style-wide,
 	position: relative;
 }
 
+#masthead .site-header-wrap {
+	margin-right: auto;
+	margin-left: auto;
+}
+
+@media only screen and (max-width: 559px) {
+	#masthead .site-header-wrap {
+		width: calc(100% - 32px);
+	}
+}
+
 #masthead .site-header-wrap > * {
 	max-width: 100%;
 }

morden/style.css

@@ -3508,6 +3508,17 @@ hr.wp-block-separator.is-style-wide,
 	position: relative;
 }
 
+#masthead .site-header-wrap {
+	margin-left: auto;
+	margin-right: auto;
+}
+
+@media only screen and (max-width: 559px) {
+	#masthead .site-header-wrap {
+		width: calc(100% - 32px);
+	}
+}
+
 #masthead .site-header-wrap > * {
 	max-width: 100%;
 }