RoboHelp 2019 - customising footer - Indigo skin

Hi

I am trying to add a footer that expands across the screen rather than limited to the iframe class="topic" area when using the Indigo skin.

For example:

I want to add a branded footer to the following area and I cannot find a way of customising the area apart from adding the code post Help build.

<div class="topicpage-footer footer" role="contentinfo" data-html="@11361180_LNG.Copyright"></div>

Can anyone help please?

Thanks

Michelle