Tesseract – resizing background image for different screen sizes

Hi,

I’m in the process of setting up a website using the tesseract theme. It looks fine on my screen, (although I need to finish it), but the background isn’t the right size for my work computer so it cuts off.

I have a fullwidth background image. Here is my site

https://aimlesswanderer.net/

How can I adjust it so it resizes to any screen?
I am also using google fonts to change the header fonts on my home PC but at work the fonts are not applied. Why would this be?

Thanks in advance.