add sass function that convert a pixel value list to rem values

app/assets/stylesheets/content/_forms.sass

@@ -379,10 +379,20 @@ fieldset.form--fieldset
 #{$text-input-selectors}
   line-height: 1.5
 
+
+@function rem-concat-list($list-of-pixels)
+  $result: ''
+  @each $pixel-val in $list-of-pixels
+    $result: $result + rem-calc($pixel-val) + ' '
+  @return #{$result}
+
+
+$select-padding: 3px, 24px, 3px, 3px
+
 .form--select
   @extend %input-style
   line-height: normal
-  padding: 3px 24px 3px 3px
+  padding: rem-concat-list($select-padding)
 
   &[multiple]
     background-image: none