Issue #2298821 by LewisNyman: Move generic layout styling into system.admin.css.

core/modules/system/css/system.admin.css

@@ -3,6 +3,47 @@
  * Styles for administration pages.
  */
 
+/**
+ * Reusable layout styles.
+ */
+.layout-container {
+  margin: 0 1.5em;
+}
+.layout-container:after {
+  content: "";
+  display: table;
+  clear: both;
+}
+
+@media screen and (min-width: 38em) {
+  .layout-container {
+    margin: 0 2.5em;
+  }
+  .layout-column {
+    float: left;  /* LTR */
+    box-sizing: border-box;
+  }
+  [dir="rtl"] .layout-column {
+    float: right;
+  }
+  .layout-column + .layout-column {
+    padding-left: 10px; /* LTR */
+  }
+  [dir="rtl"] .layout-column + .layout-column {
+    padding-right: 10px;
+    padding-left: 0;
+  }
+  .layout-column.half {
+    width: 50%;
+  }
+  .layout-column.quarter {
+    width: 25%;
+  }
+  .layout-column.three-quarter {
+    width: 75%;
+  }
+}
+
 /**
  * Administration blocks.
  */

core/themes/seven/css/layout/layout.css

@@ -1,41 +1,3 @@
-.layout-container {
-  margin: 0 1.5em;
-}
-.layout-container:after {
-  content: "";
-  display: table;
-  clear: both;
-}
-
-@media screen and (min-width: 38em) {
-  .layout-container {
-    margin: 0 2.5em;
-  }
-  .layout-column {
-    float: left;  /* LTR */
-    box-sizing: border-box;
-  }
-  [dir="rtl"] .layout-column {
-    float: right;
-  }
-  .layout-column + .layout-column {
-    padding-left: 10px; /* LTR */
-  }
-  [dir="rtl"] .layout-column + .layout-column {
-    padding-right: 10px;
-    padding-left: 0;
-  }
-  .layout-column.half {
-    width: 50%;
-  }
-  .layout-column.quarter {
-    width: 25%;
-  }
-  .layout-column.three-quarter {
-    width: 75%;
-  }
-}
-
 /**
  * Add spacing to bottom of pages
  */