You need to add this code to your child theme’s stylesheet:
@media only screen and (min-width: 800px) {
#branding {
width: 42.85%;
max-width: 450px;
}
#menu-secondary {
width: 57.15%;
max-width: 575px;
}
}
You need to adjust the width value and max-width value for #menu-secondary. However, whatever you add to that, you need to subtract from the #branding values.
If you need further help with the theme, I encourage you to check out my professional support forum at Theme Hybrid.