style: update kbd styles.

Author: jaywcjlove

src/styles/markdown.less

@@ -184,11 +184,12 @@
     border: 1px solid #d1d5da;
     border-radius: 3px;
     box-shadow: inset 0 -1px 0 #d1d5da;
-    color: #24292e;
+    color: #56595d;
     display: inline-block;
-    font-size: 11px;
+    font-size: 10px;
+    font-family: ui-monospace,SFMono-Regular,SF Mono,Consolas,Liberation Mono,Menlo,monospace;
     line-height: 10px;
-    padding: 3px 5px;
+    padding: 2px 4px;
     vertical-align: middle;
   }
 }