Hello, faced the same issue recently, 
The solution for me was to activate developer mode in the settings App.

Afterward, go back to the Settings App, under Technical, and scroll down to Views Under User Interface...

 From here, I searched for the survey in the search bar, the views here are what I experimented with to add in-line styling to my survey. 

However, I would advise that you inherit as stated in this thread and change the design accordingly. 

I am new to Odoo so I did it straight within the view but I would not advice that.

you may consider to inherit Survey Layout view and change the design accordingly