I have signed up to the Trial account of Discourse and created a custom theme.
It is working fine on the desktop, however not working on mobile and is loading entirely different HTML.
Mobile and desktop view use different HTML templates for some pages, so you’ll need to style them separately. Within your theme you can have mobile or desktop specific stylesheets, the structure of theme CSS is covered here: Structure of themes and theme components
You can also toggle mobile view on desktop to make development easier