Rename govuk-button-colour-darken-15

Rename govuk-button-colour-darken-15 to govuk-button-shadow-colour to better communicate how it is used, and decouple the name from the value. Currently to change the button shadow colour we would also have to change the name of the variable.

src/components/button/_button.scss

@@ -28,7 +28,7 @@
     outline: 1px solid transparent; // keep some button appearance when changing colour settings in browsers
     outline-offset: -1px; // fixes bug in Safari that outline width on focus is not overwritten, is reset to 0 on focus in govuk_template
     background-color: $govuk-button-colour;
-    box-shadow: 0 $button-shadow-size 0 $govuk-button-colour-darken-15; // s0
+    box-shadow: 0 $button-shadow-size 0 $govuk-button-shadow-colour; // s0
     font-family: $govuk-font-stack;
     text-align: center;
     text-decoration: none;
@@ -132,9 +132,9 @@
 
     &:active {
       top: 0;
-      box-shadow: 0 $button-shadow-size 0 $govuk-button-colour-darken-15; // s0
+      box-shadow: 0 $button-shadow-size 0 $govuk-button-shadow-colour; // s0
       @include ie-lte(8) {
-        border-bottom: $button-shadow-size solid $govuk-button-colour-darken-15; // s0
+        border-bottom: $button-shadow-size solid $govuk-button-shadow-colour; // s0
       }
     }
   }
@@ -165,9 +165,9 @@
 
     background-repeat: no-repeat;
     background-position: 100% 50%;
-    box-shadow: 0 $button-shadow-size 0 $govuk-button-colour-darken-15;
+    box-shadow: 0 $button-shadow-size 0 $govuk-button-shadow-colour;
     @include ie-lte(8) {
-      border-bottom: $button-shadow-size solid $govuk-button-colour-darken-15;
+      border-bottom: $button-shadow-size solid $govuk-button-shadow-colour;
     }
   }
 

src/globals/scss/settings/_colours-applied.scss

@@ -44,4 +44,4 @@ $govuk-border-colour: $govuk-grey-2;
 
 $govuk-button-colour: #00823b;
 $govuk-button-hover-colour: darken($govuk-button-colour, 5%);
-$govuk-button-colour-darken-15: darken($govuk-button-colour, 15%);
+$govuk-button-shadow-colour: darken($govuk-button-colour, 15%);