Hmmm just did a quick search for bg2.png in the template however it's not popping up any results. It could be in the style properties but I'm not having any luck finding it. I'll keep looking.
Think I've found it

#SignupButton .inner {
background-attachment: scroll;
background-clip: border-box;
background-color: #E03030;
background-image: url("styles/core/xenforo/images/bg2.png");
background-origin: padding-box;
background-position: 0 0;
background-repeat: repeat;
background-size: auto auto;
border-bottom-left-radius: 4px;
border-bottom-right-radius: 4px;
border-top-left-radius: 4px;
border-top-right-radius: 4px;
color: #FFFFFF;
display: block;
font-family: Calibri,'Trebuchet MS',Verdana,Geneva,Arial,Helvetica,sans-serif;
font-size: 12pt;
font-weight: bold;
text-shadow: 0 0 0 transparent, 0 0 3px rgba(0, 0, 0, 0.5);
Thanks creaky, now I recall, it was suppose to add some texture to the signup now button :) I'll get it fixed in the next version, until now:

Appearance -> Style Properties -> Buttons -> Signup inner

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