New to open core..How do i customize the black background in gui theme?

Hi,

So i'm new to open core. I'm running the latest 0.6.3... I been a clover user for years, i would revert back to clover if i could but it doesn't work well with big sur so here i am.. How do i change the gui theme background? Default is black, i would like to customize it, possibly if i can the same way i did with clover...