Adjust Responsiveness/width

Hello,

I’m trying to adjust the width of the site. I have it as boxed width, but I need it to be reduced to a max width of 900px.

Also, when I scale down to the first point that the menu changes into a mobile/stacked menu, I can’t find a way to change the colour of the menu button (3 stacked lines). Along with that, I need to adjust at what point the menu changes. It changes to the mobile menu too soon for what I need.

Please let me know how to make these adjustments. I’ve already adjusted colours everywhere else and am very competent with html & css.

Thank you!