Hey I can help with the first question:
https://colorlibsupport.com/t/remove-space-between-menu-and-body/
Just fixed mine 
Just make sure you follow instructions directly… do it through Admin Area → Appearance → Customize → Additional CSS
For some reason if you do it in the Dashboard > Appearance > Custom CSS it doesn’t function in the same manner.
That eliminates your second question and as per your third question. You can adjust the color via the visual editor in custom CSS so Dashboard > Appearance > Custom CSS – on the upper right hand corner there’s an icon of an eye, click that. Then hover over the element you want to adjust – it default loads your homepage, so type in whatever page your button is on manually – but you can make adjustments here.
I also posted a question right after you, if you have any ideas, would love some advice haha. Cheers!