MDL-48277 css: enable duplicate definitions lint rule

And remove duplicates found in code which were detected by this rule
(leaving the last rule defined as this would take precidence).

https://github.com/CSSLint/csslint/wiki/Disallow-duplicate-properties

blocks/community/styles.css

@@ -67,7 +67,6 @@
     font-weight:normal;
     line-height:1.2em;
     font-size: 140%;
-    color: #fff;
     width: 102%;
     text-indent: 15px;
 }