[Aros]

I don't know which border you are referring to, please clarify.

You can adjust the width for that area in your Style Sheet (All Style Options) Page; "Main Table Width (pixels or percentage)", but please keep in mind that it's a global setting - It will change the width of your vBulletin Skin all around.
The other option is to change the width of that table alone, in your 'footer' template;

Search for:

<table cellpadding="$stylevar[cellpadding]" cellspacing="0" border="0" width="$stylevar[outertablewidth]" class="page" align="center">

You can change the width there to for example a width of 100%;

<table cellpadding="$stylevar[cellpadding]" cellspacing="0" border="0" width="100%" class="page" align="center">