Use a media query to set different values for variables in dark-mode.
:root {
--background-color: #eee;
--text-color: #222;
}
@media (prefers-color-scheme: dark) {
:root {
--background-color: #222;
--text-color: #eee;
}
}
body {
background-color: var(--background-color);
color: var(--text-color);
}