Change RGB of Vint Age theme background

Hi,
I am very new to WP and am building my first site for a friend: https://szeles.com

I am having a hard time changing the dark grey theme background to the same black that I have used in my image assets, which is 19 / 19 / 19. When displaying across several browsers, sometime the grey is very apparent, sometimes it’s the same – and it’s bugging the heck out of me. My guess is that I have to edit the CSS somewhere. I have tried to figure it out on my own, but I am frustrating myself and can’t seem to find much help on the theme (Go Daddy/cyberchimp: Vint Age) anywhere.

Any help or links to help are greatly appreciated ??

Thanks
~Paula