FirmaSite theme is using Bootstrap responsive feature. I suggest you to read that linked article about what is responsive and how our feature working. Its a good feature but what is solution if you dont want too much wide site for Large displays (1200px and up)? Its actually simple:
@media (min-width:1200px){ .container{max-width:970px} .site-only-content .container{max-width:756px} }
You can use those css codes in your Custom Css setting. This will make your site’s width fixed for Default (992px and up) and Large displays (1200px and up).
Thank you very much, perfect.