My Project Looks Very Different When I Select 'Open in Browser'

Long story short, while doing the design itself, it looks fine, however after deploying (to docker localhost/home), it looks naked, bare html with plain text and grey buttons, blue links etc.

Am I missing something here? In the right-hand ‘Design’ tab it says ‘No user style file attached’, however I’m starting my first project, I don’t have any user style file to upload or attach.

Previously when I created a demo project and clicked ‘Open in Browser’, i was able to see the full css design and style, so I assumed that what I dragged and drog in the main Design Tab would reflect directly on the Open in Browser view?