Update to jekyll 4

_sass/_base.scss

@@ -120,7 +120,7 @@ a {
     color: $brand-color;
     text-decoration: none;
     &:visited {
-        color: darken($brand-color, 15%);
+        color: color.adjust($brand-color, $lightness: -15%, $space: hsl);
     }
     &:hover {
         color: $text-color;