Semantic UI centre aligned login box?
<div class="ui column stackable center page grid">
<div class="four wide column"></div><!-- empty div just padding -->
<form class="ui six wide column form segment">
<!-- login input fields go here -->
</form>
</div>
See this code found in this link:
<div class="ui one column stackable center aligned page grid">
<div class="column twelve wide">
Your stuff here
</div>
</div>