ID-473 Fix wcag test.

Currently relies on apply-brand setting some LESS variables. No harm in setting them for now, even though they're not used for anything else

Co-authored-by: Copilot <copilot@github.com>

Author: halfninja

less/hp-2025/site.less

@@ -1,7 +1,7 @@
 @import (reference) '../id7.less';
 @import (reference) 'hp.less';
 
-.apply-brand(@brand-colour);
+.apply-css-variables(@brand-colour);
 
 .id7-utility-masthead {
   background-color: @brand-colour;

less/mixins/branding.less

@@ -178,6 +178,10 @@
   - Mix in the id7-brand-COLOURNAME class in your own build if adding it to the HTML document isn't an option
   - If you want to use a custom colour, call .brand-css-variables(COLOR) inside :root
 */
+  // These are still in here for now for the tests to check .calculate-brand-colours is working correctly, 
+  // but they don't generate output.
+  @colours: .calculate-brand-colours(@colour);
+  @link-colour: @colours[@white-emphasis];
 }