Use _govuk-brand-colours helper to set brand colours currently used

Author: romaricpascal

packages/govuk-frontend/src/govuk/components/service-navigation/_index.scss

@@ -18,7 +18,7 @@
     @include _govuk-rebrand(
       "background-color",
       $from: $govuk-service-navigation-background,
-      $to: $_govuk-rebrand-template-background-colour
+      $to: _govuk-brand-colour("blue", "tint-95")
     );
   }
 

packages/govuk-frontend/src/govuk/settings/_colours-applied.scss

@@ -184,10 +184,10 @@ $govuk-link-active-colour: govuk-colour("black") !default;
 ///
 /// @type Colour
 /// @access private
-$_govuk-rebrand-template-background-colour: govuk-tint($govuk-brand-colour, 95%);
+$_govuk-rebrand-template-background-colour: _govuk-brand-colour("blue", "tint-95");
 
 /// Border colour for areas on a light-blue background
 ///
 /// @type Colour
 /// @access private
-$_govuk-rebrand-border-colour-on-blue-tint-95: govuk-tint($govuk-brand-colour, 50%);
+$_govuk-rebrand-border-colour-on-blue-tint-95: _govuk-brand-colour("blue", "tint-50");