header width

Hi all, newbie here, just building out my first forum on Xenforo cloud. Know a little CSS but not a lot. I've been setting up the page basics, and noticed that under 650px, my header row vanishes and the logo moves to the Nav bar. I'm not keen on this and would prefer the row and logo to stay...

I'm working on a custom style. The header and secondary navbar extend to the full width, but I'd like to limit them to the inner width displayed. I've spent a while exploring inspect > element and customizing the CSS with no dice. Can anyone point me in the right direction? See attachment.