bootstrap-grid.css: Only apply box-sizing: border-box to the columns instead of using the universal selector

scss/mixins/_grid.scss

@@ -10,6 +10,8 @@
 }
 
 @mixin make-col-ready($gutter: $grid-gutter-width) {
+  // Add box sizing if only the grid is loaded
+  box-sizing: if(variable-exists(include-column-box-sizing) and $include-column-box-sizing, border-box, null);
   // Prevent columns from becoming too narrow when at smaller grid tiers by
   // always setting `width: 100%;`. This works because we set the width
   // later on to override this initial width.