Can you specify which exactly do you want styled on mobile?
If it’s the thrive lead forms, that’s something out of the theme’s scope of support as the theme doesn’t control how it is styled.
But to help you out:
I worked for Thrive for a few months last year, and if I remember it right, Thrive Leads form is styled within its plugin’s UI. I believe it’s using Thrive architect as shown here – https://share.getcloudapp.com/8Luo2OnJ – which has its own responsive toggle for different styles on different viewports.