EDUCATOR-3914 fix enroll now text contrast ratio on LMS

lms/static/sass/bootstrap/_components.scss

@@ -17,8 +17,13 @@
 
     li {
       margin: 5px 0;
+
+      .alert.alert-warning .message-content .enroll-btn.btn-link {
+        color: #004368;
+      }
+
     }
-  }
+  } 
 
 }