Take a look around line 35 in skin1/mail/html/order_invoice.tpl
I have an old X-cart version, maybe yours not the same....
Code:
{config_load file="$skin_config"}
{literal}
<STYLE type="text/css">
BODY {
MARGIN-TOP: 10px;
FONT-SIZE: 12px;
MARGIN-LEFT: 10px;
FONT-FAMILY: arial,helvetica,sans-serif
}
TD {
FONT-SIZE: 12px;
FONT-FAMILY: arial,helvetica,sans-serif
}
TH {
FONT-SIZE: 13px;
FONT-FAMILY: arial,helvetica,sans-serif
}
H1 {
FONT-SIZE: 20px
}
</STYLE>
{/literal}
<table cellSpacing="0" cellPadding="0" width="600" bgColor="#ffffff" border="0" color="#000000">
<tr>
<td>
<table cellSpacing="0" cellPadding="0" width="100%" border="0"><tr>
<td vAlign="top"><br>
</td>
<td width="100%">
<table cellSpacing="0" cellPadding="2" width="100%" border="0">
<tr>
<td width="30"> </td>
<td vAlign="top"><font style="FONT-SIZE: 28px"><b>INVOICE</b></font>
<br>
<br>
<b>Date:</b>