border-bottom: 3px solid #555;
border-radius: 3px;
(repeat all browser compatible radius declarations)
Okay, seems you already have the style applied and I assume that style is declared as .button.
So, find the link (probably in showthread template group). This should be like:
<a href=" ....."><i class="fa fa-lock"></i> Close</a>
Make it:
<a href=" ....." class="button"><i class="fa fa-lock"></i> Close</a>
OR:
<a href=" ....."><span class="button"><i class="fa fa-lock"></i> Close</span></a>