I've built my survey using Connect and successfully published to my ArcGIS Online account. When I make design changes (add a photo to the background, change the color from default green to blue, customize the thank you screen, etc) and hit "publish", it tells me that it was "Published Successfully!" But, when I open the survey in my browser and via the Survey123 smartphone app, none of the design changes are there; it's back to the default settings.
What version of Connect and web app are you using? It might be that you are using an earlier version of Connect. Please go to the survey123 website and click the survey>Settings>choose "Always use the latest version (3.9) and republish again. You should be able to see the updated theme and background in both web form and field app.