create vars for responsive grid values, fixed and fluid, and document them in the customizer

less/variables.less

@@ -198,6 +198,7 @@
 // GRID
 // --------------------------------------------------
 
+
 // Default 940px grid
 // -------------------------
 @gridColumns:             12;
@@ -205,7 +206,24 @@
 @gridGutterWidth:         20px;
 @gridRowWidth:            (@gridColumns * @gridColumnWidth) + (@gridGutterWidth * (@gridColumns - 1));
 
+// 1200px min
+@gridColumnWidth1200:     70px;
+@gridGutterWidth1200:     30px;
+
+// 768px-979px
+@gridColumnWidth768:      42px;
+@gridGutterWidth768:      20px;
+
+
 // Fluid grid
 // -------------------------
 @fluidGridColumnWidth:    6.382978723%;
 @fluidGridGutterWidth:    2.127659574%;
+
+// 1200px min
+@fluidGridColumnWidth1200:     5.982905983%;
+@fluidGridGutterWidth1200:     2.564102564%;
+
+// 768px-979px
+@fluidGridColumnWidth768:      5.801104972%;
+@fluidGridGutterWidth768:      2.762430939%;