Xmodule: darkening 'correct' green color for WCAG AA requirements

common/lib/xmodule/xmodule/css/capa/display.scss

@@ -23,8 +23,8 @@
 // ====================
 $annotation-yellow: rgba(255,255,10,0.3);
 $color-copy-tip: rgb(100,100,100);
-$correct: $green-d1;
-$partiallycorrect: $green-d1;
+$correct: $green-d2;
+$partiallycorrect: $green-d2;
 $incorrect: $red;
 
 // +Extends - Capa