Custom Layout in Adobe Robohelp

Assuming Robohelp 2019 New UI, you open a skin in the Robohelp and update the values provided in the editor.

There are a number of different skins for Frameless and Responsive HTML5 with different elements that will hopefully meet your needs.

RH2019 is really new, so we're all learning what's possible. If you have a specific question, post back with the skin you're customising and what you're trying to achieve, preferably with a screenshot or mockup to help us understand, and we can poke around to see if we can figure anything out.