Fix #449

sass/grid/columns.sass

@@ -204,8 +204,6 @@
         margin-left: ($i / 12) * 100%
 
 .columns
-  margin-left: -0.75rem
-  margin-right: -0.75rem
   margin-top: -0.75rem
   &:last-child
     margin-bottom: -0.75rem
@@ -237,6 +235,8 @@
           margin-left: 0
   &.is-mobile
     display: flex
+    margin-left: -0.75rem
+    margin-right: -0.75rem
   &.is-multiline
     flex-wrap: wrap
   &.is-vcentered
@@ -245,7 +245,11 @@
   +tablet
     &:not(.is-desktop)
       display: flex
+      margin-left: -0.75rem
+      margin-right: -0.75rem
   +desktop
     // Modifiers
     &.is-desktop
       display: flex
+      margin-left: -0.75rem
+      margin-right: -0.75rem