[JENKINS-71479] Do not use SCSS lighten function directly to avoid invalid CSS (#8425)

(cherry picked from commit 620e775)

Author: dwnusbaum

war/src/main/scss/abstracts/theme.scss

@@ -131,9 +131,9 @@ $semantics: (
   // Button primary
   --button-color--primary: var(--background);
   --btn-primary-bg: #063f61;
-  --btn-primary-bg-hover: lighten(#063f61, 7.5%);
-  --btn-primary-bg-active: lighten(#063f61, 12%);
-  // Button primary
+  --btn-primary-bg-hover: #{lighten(#063f61, 7.5%)};
+  --btn-primary-bg-active: #{lighten(#063f61, 12%)};
+  // Deprecated - Button primary
   --btn-secondary-color: var(--secondary);
   --btn-secondary-bg: var(--btn-text-color);
   --btn-secondary-border: var(--medium-grey);