Use hardcoded colors for fade(…) mixin

cfgov/unprocessed/apps/form-explainer/css/form-explainer.less

@@ -2,24 +2,24 @@
    "Form explainer" custom styles
    ========================================================================== */
 
-@gold-dk: var(--gold);
-@gold-md: var(--gold-60);
-@gold-lt: var(--gold-50);
+@gold-dk: #ff9e1b; // gold
+@gold-md: #ffce8d; // gold-60
+@gold-lt: #ffd8a3; // gold-50
 @gold-bg: #fff8f0;
 
-@red-dk: var(--purple-60);
-@red-md: var(--purple-50);
-@red-lt: var(--purple-20);
+@red-dk: #d486b2; // purple-60
+@red-md: #dc9cbf; // purple-50
+@red-lt: #f0d8e2; // purple-20
 @red-bg: #f9f6f6;
 
-@blue-dk: var(--navy-70);
-@blue-md: var(--navy-50);
-@blue-lt: var(--navy-20);
+@blue-dk: #6f88b2; // navy-70
+@blue-md: #9daecc; // navy-50
+@blue-lt: #d3daeb; // navy-20
 @blue-bg: #f4f7fa;
 
-@teal-dk: var(--teal-80);
-@teal-md: var(--teal-70);
-@teal-lt: var(--teal-20);
+@teal-dk: #579695; // teal-80
+@teal-md: #70a6a5; // teal-70
+@teal-lt: #d4e7e6; // teal-20
 @teal-bg: #f2f8f8;
 
 @colors: gold, red, blue, teal;