Make header background color opaque?

Yes, add the below CSS using some custom CSS plugin

#sq-masthead {
background: #FFF !important;
}

This will break the black header option if you change to black

I wanted to also use a white background colour in the header. However, as the title links in the header is also white, how do I change this to another colour instead?

Thanks for your help.

@superfishnz There is option to change the header background color to white in the customizer settings. The color will automatically change to black in the while background.

Check the General Settings -> Header Settings -> Header background color field.