style: update markdown styles.

Author: jaywcjlove

src/styles/markdown.less

@@ -10,7 +10,8 @@
     margin-bottom: 0 !important;
   }
 
-  pre[class*='language-'] {
+  pre[class*='language-'],
+  pre {
     &:hover .copied {
       visibility: visible;
     }