You need to play with the PAGE_CONTAINER, I modified it there so I can customize further to our needs:
Look for this part:
Code:
<xf:set var="$srcset">{{ property('publicLogoUrl2x') ? base_url(property('publicLogoUrl2x')) . ' 2x' : '' }}</xf:set>
<header class="p-header" id="header">
<div class="p-header-inner">
<div class="p-header-content">
<div class="p-header-logo p-header-logo--image">
<a href="{{ ($xf.options.logoLink && $xf.homePageUrl) ? $xf.homePageUrl : link('index') }}">
<img src="{{ base_url(property('publicLogoUrl')) }}" srcset="{$srcset}" alt="{$xf.options.boardTitle}"
width="{{ property('publicLogoWidth') ?: '' }}" height="{{ property('publicLogoHeight') ?: '' }}" />
</a>
</div>
<xf:ad position="container_header" />
</div>
</div>
</header>
Save a copy of the original code, then play with it to see how you'd like it customized.