TO center the login logo, just edit /skins/default/templates/login.html or /skins/larry/templates/login.html, whichever applies to your case.
Look for this at the top of the file
<div id="login-form">
<div class="box-inner">
<roundcube:object name="logo" src="/images/roundcube_logo.png" id="logo" />
<roundcube:form name="form" method="post">
<roundcube:object name="loginform" form="form" size="40" submit=true />
</form>
and replace the line <div class="box-inner"> with <center><div class="box-inner"></center>
I know that you should style with css preferably, but for a quick fix, this is what I use