X is the usergroupid - you would not leave it as X, but change it to the userid you want.
Lets say you want to change the font in the main post area. You would need to find that area. You will find it by looking for <!-- message --> in the postbit, or postbit_legacy depending on which you use, template. Let's assume you don't care about the post title and instead only the color of the post itself. In that case, we will just use the div tag right below the <!-- message --> (it's in red and it's for a 3.8 template - yours may be different):
Code:
<!-- message -->
<div id="post_message_$post[postid]">
$ad_location[ad_showthread_firstpost_start]
$post[message]
</div>
<!-- / message -->
Now you would use the conditions I posted above. Let's say you only care about administrators, groupid 6, supermods, groupid 5, and then the rest of the posters. You want admins in yellow, supermods in red, and everyone else in blue. So, you'd do this *in place of* the stuff in red above:
HTML Code:
<if condition="is_member_of($post, 6)">
<div id="post_message_$post[postid]" style="color: yellow;">
<else />
<if condition="is_member_of($post, 5)">
<div id="post_message_$post[postid]" style="color: red;">
<else />
<div id="post_message_$post[postid]" style="color: blue;">
</if>
</if>