Hi @zerocuul,
1. If you change the width, then you will need to adjust the sizes for all devices. Please hire a customizer to make it work on all devices. There will be a lot of changes. Also, consider using a child theme and its style.css as there will be quite a few changes. Custom CSS box is only meant for small changes.
2. It is possible to change color of child menu.For that, you can add following CSS code in Appearance=> Customize=> Theme Options => Custom CSS box:
#branding #access, #colophon #access-footer, #branding ul.menu ul a {
background: #CD0000;
}
Change the hex color value to any value you like.
Regards,
Pratik