Fix Sass color deprecation warnings

Author: paulrobertlloyd

app/assets/stylesheets/_button.scss

@@ -1,5 +1,5 @@
 $app-cis2-button-color: $color_nhsuk-blue;
-$app-cis2-button-hover-color: mix($app-cis2-button-color, #003087, 50%);
+$app-cis2-button-hover-color: color.mix($app-cis2-button-color, #003087, 50%);
 $app-cis2-button-active-color: #003087;
 $app-cis2-button-shadow-color: #003087;
 $button-shadow-size: 4px;

app/assets/stylesheets/_highlight.scss

@@ -1,5 +1,5 @@
 .app-highlight {
-  background-color: mix(white, $color_nhsuk-warm-yellow, 80%);
+  background-color: color.mix(white, $color_nhsuk-warm-yellow, 80%);
   border-bottom: 3px solid $color_nhsuk-warm-yellow;
 
   // Ensure highlight is announced by screen readers

app/assets/stylesheets/application.scss

@@ -29,9 +29,9 @@ $mq-breakpoints: (
 
 // App options
 $app-page-width: 1100px;
-$color_app-dark-orange: darken(
-  mix($color_nhsuk-red, $color_nhsuk-warm-yellow, 55%),
-  10%
+$color_app-dark-orange: color.scale(
+  color.mix($color_nhsuk-red, $color_nhsuk-warm-yellow, 55%),
+  $lightness: -10%
 );
 
 // Helpers