Hi and thank you!
To change the break-point you need to change media query in the style.css, in following places:
Line 886 – min-width: 1200px
Line 1248 – max-width: 1199px
Line 1298 – min-width: 1200px
Line 1708 – min-width: 1200px
So it’s not a small change that you would just drop in the customizer additional CSS or similar. If you alter main CSS you must create a minified version, because in production the style.min.css is in use. But as you may know, altering parent theme is not recommended, because any change will be lost when the theme is next updated.
Better option is use of child-theme. I created a child theme for you, with altered CSS, so that the break-point for mobile menu is at 900px. To change the break point you have to alter one line – L 18 (first line of CSS, right after the style comment). Only possible downside is that this work only for breakpoints below 1200px, but I guess that’s what you want.
Download it here https://relishpress.com/wp-content/uploads/2017/06/themelia-child.zip and let me know how this work for you.