Extend spacers and sizes (#25101)

scss/_variables.scss

@@ -114,23 +114,27 @@ $enable-print-styles:       true !default;
 // variables. Mostly focused on spacing.
 // You can add more entries to the $spacers map, should you need more variation.
 
+// stylelint-disable
 $spacer: 1rem !default;
-$spacers: (
+$spacers: () !default;
+$spacers: map-merge((
   0: 0,
   1: ($spacer * .25),
   2: ($spacer * .5),
   3: $spacer,
   4: ($spacer * 1.5),
   5: ($spacer * 3)
-) !default;
+), $spacers);
 
 // This variable affects the `.h-*` and `.w-*` classes.
-$sizes: (
+$sizes: () !default;
+$sizes: map-merge((
   25: 25%,
   50: 50%,
   75: 75%,
   100: 100%
-) !default;
+), $sizes);
+// stylelint-enable
 
 // Body
 //