Pon lo siguiente al final del archivo index.css:
.main { width:400px; height:250px; position:absolute; top:50%; left:50%; margin-top:-125px; margin-left:-200px;}
.box { width:370px; overflow:hidden; padding:15px; background:#fff; border:1px solid #d5d5d5; border-radius: 10px; -moz-border-radius: 10px; -webkit-border-radius: 10px; box-shadow: 0 0 20px rgba(3,44,52,.22); -moz-box-shadow: 0 0 20px rgba(3,44,52,.22); -webkit-box-shadow: 0 0 20px rgba(3,44,52,.22);}
.box h2 { color:#292929; font-size:25px; margin:0; padding:0 0 10px 0; font-weight:bold; outline:none;}
.box h3 { color:#7f7f7f; font-size:17px; line-height:20px; margin:0; padding:0; font-weight:normal; outline:none;}
.form { margin:0; padding:0;}
.form fieldset { border:none; padding: 10px 0 0; margin:0;}
.form .row { margin-bottom:6px; position:relative; line-height:33px;}
.form .row input[type="text"], .form .row input[type="password"] { height:33px; line-height:33px; border:1px solid #c5c5c5; border-radius:3px; -moz-border-radius:3px; -webkit-border-radius:3px; box-shadow: 0 0 5px rgba(0,0,0,.1), 0 2px 2px rgba(0,0,0,.1) inset; -moz-box-shadow: 0 0 5px rgba(0,0,0,.1), 0 2px 2px rgba(0,0,0,.1) inset; -webkit-box-shadow: 0 0 5px rgba(0,0,0,.1), 0 2px 2px rgba(0,0,0,.1) inset;}
.form .row input[type="text"]:focus, .form .row input[type="text"]:hover, .form .row input[type="password"]:focus, .form .row input[type="password"]:hover { border:1px solid #b8d564; box-shadow: 0 0 5px #b8d564, 0 2px 2px rgba(0,0,0,.1) inset; -moz-box-shadow: 0 0 5px #b8d564, 0 2px 2px rgba(0,0,0,.1) inset; -webkit-box-shadow: 0 0 5px #b8d564, 0 2px 2px rgba(0,0,0,.1) inset;}
.form .row input.login { background:url(../images/input_login.png) left center no-repeat #f5f5f5; width:100%; padding: 1px 5px 1px 45px;}
.form .row input.password { background:url(../images/input_password.png) left center no-repeat #f5f5f5; padding: 1px 130px 1px 45px; }
.form .row input.error { border:1px solid #eb9393; box-shadow: 0 0 5px #f7c4c4; -moz-box-shadow: 0 0 5px #f7c4c4; -webkit-box-shadow: 0 0 5px #f7c4c4; background-color:#fce4e4; }
.form .row a.forgot { position:absolute; top:2px; right:10px; color:#909090; font-size:12px; text-shadow:1px 1px 1px #fff;}
.form .row a.forgot:hover { color:#6e6e6e;}
.form .row input[type="checkbox"] { display:block; float:left; margin: 9px 6px 9px 2px; }
.form .row label { color:#086f86; font-size:14px; display:block; float:left; }
.form .row input[type="submit"] { cursor:pointer; color:#1c464e; text-shadow:1px 1px 0 #aaeaf7; border:1px solid #289cb8; border-radius:3px; -moz-border-radius:3px; -webkit-border-radius:3px; background:#67bbcf; background: -webkit-gradient(linear, bottom, top, from(#67bbcf), to(#82def1)); background: -moz-linear-gradient(bottom, #67bbcf, #82def1); font-size:16px; padding:5px 20px; font-weight:bold; float:right;}
.form .row input[type="submit"]:hover { background:#51b1c7; background: -webkit-gradient(linear, bottom, top, from(#4297ab), to(#60cce3)); background: -moz-linear-gradient(bottom, #4297ab, #60cce3);}
.copy { float:right; color:#9eabae; font-size:12px; margin:10px 16px 0 0; text-shadow:1px 1px 1px #fff;}
Luego, sube todas las imagenes que vienen allí en /images dejándolas en el siguiente directorio: /foro/Themes/el-tema-que-usas/images/
Por último, busca en el Index.template.php la parte de <body> para que inmediatamente después de ello agregues:
<div class="main">
<div class="box">
<h2>Ingreso de cuenta</h2>
<h3><a href="./register.php">Regístrate</a> o ingresa tu usuario y contraseña</h3>
<form class="form" action="validar.php" method="post">
<fieldset>
<div class="row">
<input type="text" class="login" name="nick" placeholder="Usuario" />
</div>
<div class="row">
<input type="password" class="password" name="password" placeholder="Password"/>
<a class="forgot" href="#">Olvide mi password</a>
</div>
<div class="row">
<input type="checkbox" class="remember" name="remember" id="remember" />
<label for="remember">Mantenerme logeado</label>
<input type="submit" value="Ingresar" name="Ingresar"/>
</div>
</fieldset>
</form>
</div>
<span class="copy">Copyright © 2012 iNews System</span> </div>