[Pages] tuning index style

Signed-off-by: Eric Wang <skygragon@gmail.com>

Author: skygragon

docs/index.html

@@ -2,6 +2,10 @@
 layout: default
 title: Leetcode CLI
 ---
-
-<a href="./commands" class="btn">Commands Tutorials</a>
-<a href="./advanced" class="btn">Advanced Topics</a>
+<style>
+  .btn-clear {color: rgba(0, 0, 0, 0.5); border: 1px solid;}
+</style>
+<center>
+  <a href="./commands" class="btn btn-clear">Commands Tutorials</a>
+  <a href="./advanced" class="btn btn-clear">Advanced Topics</a>
+</center>