<p>Pure CSS background pattern. Can you figure out how it was made before checking the code? Can you reduce the number of gradients? Let me know!</p>

$d: 6.25em;

html {
  --g: 
    radial-gradient(closest-side, 
        #fff calc(100% - 1px), #000);
  background: 
    repeating-conic-gradient(from 45deg, 
        #fff 0% 25%, #000 0% 50%) .5*$d 0, 
    linear-gradient(#a1c181 0 0), 
    var(--g), var(--g) .5*$d .5*$d;
  background-size: $d $d;
  background-blend-mode: difference, darken;
}