Preview:
This is only the header and the header_welcomeblock_guest, if you want to completely create a seamless facebook login page, you need to combine this with the registration of MyBB and put that template first (like the index).
Modifications:
header template:
<div id="header">
<div class="wrapper">
<div class="logo">
<a href="{$mybb->settings['bburl']}/index.php"><img src="{$theme['logo']}" alt="{$mybb->settings['bbname']}" title="{$mybb->settings['bbname']}" /></a>
</div>
<div class="user">
{$welcomeblock}
</div>
</div>
</div>
<br />
header_welcomeblock_guest template (This isn't the best way to achieve this, but it's late here, so I use tables, sorry):
<div class="login">
<form action="member.php" method="post">
<table border="0" cellspacing="0" cellpadding="8">
<tr>
<td style="margin: 0; padding: 0;"><span class="text">Email or Phone</span></td>
<td style="margin: 0; padding: 0 0 0 8px;"><span class="text">Password</span></td>
<td rowspan="3"><input class="loginbutton" type="submit" name="submit" value="Log In" style="cursor: pointer;" /></td>
</tr>
<tr>
<td style="padding: 0;"><input type="text" class="textlog" name="username" size="25" maxlength="{$mybb->settings['maxnamelength']}" value="{$username}" /></td>
<td style="margin: 0;"><input type="password" class="textlog" name="password" size="25" value="{$password}" /></td>
</tr>
<tr>
<td style="margin: 0; padding: 0;">
<input name="quick_remember" id="quick_login_remember" type="checkbox" value="yes" class="checkbox" checked="checked" style="float: left; margin: 0; padding: 0;" />
<label for="quick_login_remember" style="color: #789abb; font-size: 11px; vertical-align: text-bottom; float: left; text-align: left; margin: 0 0 0 5px; padding: 0; line-height: 11px;">Keep me logged in</label>
</td>
<td style="margin: 0; padding: 0;">
<a href="{$mybb->settings['bburl']}/member.php?action=lostpw" style="color: #789abb; font-size: 11px; vertical-align: text-bottom; float: left; text-align: left; margin: 0 0 0 9px; padding: 0; line-height: 11px;">Forgot your password?</a>
</td>
</tr>
</table>
<input type="hidden" name="action" value="do_login" /><input type="hidden" name="url" value="{$redirect_url}" />
</form>
</div>
CSS:
#header {
width: 100%;
min-width: 100%;
height: 120px;
background: #3a5795;
border-bottom: 1px solid #1c3674;
box-shadow: 0 0 2px #161616;
}
.logo {
float: left;
margin: 24px 0 0 0;
}
.user {
float: right;
margin: 24px 0 0 0;
}
.login {
height: 44px;
line-height: 44px;
margin-right: 10px;
}
.login ul {
float: right;
margin: 0;
list-style: none;
}
.login li {
display: inline;
}
.textlog {
font-size: 11px;
width: 142px;
color: #000000;
background: #FFFFFF;
border: 1px solid #1d2a5b;
padding: 4px 0;
margin: 0;
float: left;
}
.text {
float: left;
text-align: left;
color: #FFFFFF;
margin: 0 0 0 1px;
padding: 0;
line-height: 7px;
font-size: 11px;
}
input.loginbutton {
font-family: Tahoma;
font-size: 11px;
font-weight: bold;
color: #FFFFFF;
line-height: 13px;
padding: 5px;
margin: 0 0 6px 0;
border: 1px solid #2f477a;
box-shadow: 0 1px 0 rgba(0, 0, 0, 0.1);
background: #8a9cc2; /* Old browsers */
background: -moz-linear-gradient(top, #8a9cc2 0%, #8a9cc2 0%, #8a9cc2 3%, #637bad 3%, #5972a7 100%); /* FF3.6+ */
background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,#8a9cc2), color-stop(0%,#8a9cc2), color-stop(3%,#8a9cc2), color-stop(3%,#637bad), color-stop(100%,#5972a7)); /* Chrome,Safari4+ */
background: -webkit-linear-gradient(top, #8a9cc2 0%,#8a9cc2 0%,#8a9cc2 3%,#637bad 3%,#5972a7 100%); /* Chrome10+,Safari5.1+ */
background: -o-linear-gradient(top, #8a9cc2 0%,#8a9cc2 0%,#8a9cc2 3%,#637bad 3%,#5972a7 100%); /* Opera 11.10+ */
background: -ms-linear-gradient(top, #8a9cc2 0%,#8a9cc2 0%,#8a9cc2 3%,#637bad 3%,#5972a7 100%); /* IE10+ */
background: linear-gradient(to bottom, #8a9cc2 0%,#8a9cc2 0%,#8a9cc2 3%,#637bad 3%,#5972a7 100%); /* W3C */
filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#8a9cc2', endColorstr='#5972a7',GradientType=0 ); /* IE6-9 */
}
input.loginbutton:active {
background: #4e69a2;
border-bottom-color: #2f477a;
box-shadow: inset 0 1px 1px rgba(0, 0, 0, 0.2);
outline: none;
}