Fix glitch high-contrast skin text colors

Port from 3c5006ec7f

app/javascript/flavours/glitch/styles/contrast/variables.scss

@@ -4,6 +4,7 @@ $black: #000000;
 $classic-base-color: #282c37;
 $classic-primary-color: #9baec8;
 $classic-secondary-color: #d9e1e8;
+$classic-highlight-color: #2b90d9;
 
 $ui-base-color: $classic-base-color !default;
 $ui-primary-color: $classic-primary-color !default;
@@ -15,6 +16,7 @@ $ui-highlight-color: #2b5fd9;
 $darker-text-color: lighten($ui-primary-color, 20%) !default;
 $dark-text-color: lighten($ui-primary-color, 12%) !default;
 $secondary-text-color: lighten($ui-secondary-color, 6%) !default;
+$highlight-text-color: $classic-highlight-color !default;
 $action-button-color: #8d9ac2;
 
 $inverted-text-color: $black !default;