You can do this in the CSS or via templates.
CSS:
Templates & Style > Themes > Default > g33k_thankyoulike.css > Advanced
Replace with:
div[id^=tyl_btn_] {
display: inline-block;
}
a.add_tyl_button span{
background-image: none;
margin-left: -20px;
font-weight: bold;
}
a.add_tyl_button span:before {
content: "\f164";
font-family:FontAwesome;
}
a.del_tyl_button span{
background-image: none;
margin-left:-20px;
font-weight: normal;
}
a.del_tyl_button span:before {
content: "\f165";
font-family:FontAwesome;
}
.tyllist{
background-color: #f5f5f5;
border-top: 1px dotted #ccc;
border-bottom: 1px dotted #ccc;
padding: 2px 5px;
}
.tyllist_classic{
background-color: #f5f5f5;
border-top: 1px dotted #ccc;
border-bottom: 1px dotted #ccc;
padding: 2px 5px;
}
img[id^=tyl_i_expcol_]{
vertical-align: bottom;
}
Templates:
Edit template: thankyoulike_button_add
Replace with:
<div id="tyl_btn_{$post['pid']}" class="postbit_buttons"><a class="add_tyl_button" href="thankyoulike.php?action=add&pid={$post['pid']}&my_post_key={$mybb->post_code}" onclick="return thankyoulike.add({$post['pid']}, {$post['tid']});" title="{$lang->add_tyl}" id="tyl_a{$post['pid']}"><span id="tyl_i{$post['pid']}" style="background-image:none;margin-left:-20px;"><i class="fa fa-thumbs-up fa-fw"></i>{$lang->add_tyl}</span></a></div>
Edit template: thankyoulike_button_del
Replace with:
<div id="tyl_btn_{$post['pid']}" class="postbit_buttons"><a class="del_tyl_button" href="thankyoulike.php?action=del&pid={$post['pid']}&my_post_key={$mybb->post_code}" onclick="return thankyoulike.del({$post['pid']}, {$post['tid']});" title="{$lang->del_tyl}" id="tyl_a{$post['pid']}"><span id="tyl_i{$post['pid']}" style="background-image:none;margin-left:-20px;"><i class="fa fa-thumbs-down fa-fw"></i>{$lang->del_tyl}</span></a></div>
Please note: With the templates, I've overwrited some CSS properties (background-image and margin-left), you can modify these to your own needs or even modify it in the CSS file itself, rather than inline.