If you want to change the point at which your site switches from desktop to mobile nav, this is what worked for me based on the above:
You cannot amend this using the custom CSS in wordpress, you have to amend the main Roseta stylesheet which you can find at $yourwebsite/wp-content/themes/roseta/style.css
Make a copy of it on your server, then download it via FTP. To make the change search for all instances of 800px and replace 800 with whatever width you’d like the menu to change at. The best way I’ve found to find out the px size you need is using chrome.
Right-click on the page, and click inspect which will open up the dev tools toolbar. Take chrome out of full-screen mode, then grab a side of the screen and start dragging it smaller. You will then see in the top right of the website window a width and height value appears. Drag the window smaller to see where you start to have issues, then use that value instead of 800.
I hope that helps