fix mixin bug

Author: kkartunov

src/shared/components/Contentful/Article/Article.jsx

@@ -338,7 +338,7 @@ class Article extends React.Component {
               </div>
               {/* Discord */}
               <div className={theme.actionContainer}>
-                <a className={theme['primary-green-md']} href="https://discord.gg/topcoder?ref=thrive-article" rel="noopener noreferrer" target="_blank"><DiscordIconWhite /> Chat on Discord</a>
+                <a className={theme.primaryGreenMd} href="https://discord.gg/topcoder?ref=thrive-article" rel="noopener noreferrer" target="_blank"><DiscordIconWhite /> Chat on Discord</a>
               </div>
             </div>
           </div>

src/shared/components/Contentful/Article/themes/default.scss

@@ -7,7 +7,7 @@
   flex-direction: column;
 }
 
-.primary-green-md {
+.primaryGreenMd {
   outline: none;
 
   @include primary-green;