LMS: adjusting Wiki alert styles

lms/static/sass/course/wiki/_wiki.scss

@@ -896,11 +896,13 @@ section.wiki {
 
   .alert {
     position: relative;
+    width: auto;
     margin: 24px 40px;
     padding: 8px 12px;
     border: 1px solid #EBE8BF;
     border-radius: 3px;
     background: $yellow;
+    color: $black;
     font-size: 0.9em;
 
     .close {
@@ -908,7 +910,7 @@ section.wiki {
       right: 12px;
       font-size: 1.3em;
       top: 6px;
-      color: #999;
+      color: $black;
       text-decoration: none;
     }
   }