Remove borders from menubuttons.
Or at the very least they should be centerd and square. The back button especially looks out of place.
And for the backbutton you can add css to emulate this. Here is the reference css https://github.com/elementary/stylesheet/blob/fca53948125f8102f5c66cadd8a818fa2eacaec2/elementary/gtk-3.0/granite-widgets.css#L41