TL;DR; use a negative GRAD value

GRAD stands for "grade", and it's a variable font axis that allows us to adjust the perceived weight of a font without changing the glyph size.

GRAD stands for "grade", and it's a variable font axis that allows us to adjust the perceived weight of a font without changing the glyph size.

body {
  --GRAD: 0;
  font-variation-settings: "GRAD" var(--GRAD);
}

@media (prefers-contrast: more) { body { --GRAD: 700 } }
@media (prefers-contrast: less) { body { --GRAD: -50 } }

@media (prefers-color-scheme: dark)                              { body { --GRAD: -50 } }
@media (prefers-color-scheme: dark) and (prefers-contrast: more) { body { --GRAD: 150 } }
@media (prefers-color-scheme: dark) and (prefers-contrast: less) { body { --GRAD: -150 } }