+gutter and gutter-half setting

"Gutters or alleys are spacing between content tracks".

For more info see the Mozilla Developer Network Docs https://developer.mozilla.org/en-US/docs/Glossary/Gutters

src/scss/_variables.scss

@@ -4,6 +4,9 @@
 
 $container-page-width: 1200px !default;
 
+$gutter: 30px;
+
+$gutter-half: $gutter / 2;
 
 // colours
 $darkgray: #000000;