Set our theme in seperate .css files:

:root {
    --background: #18181B;
    --foreground: #FFFFFF;
    --highlights: #FF0000;
}

Include our stylesheets based on color-scheme:

<link
  rel="stylesheet"
  href="/static/css/dark.css"
  media="(prefers-color-scheme: dark)"
/>

<link
  rel="stylesheet"
  href="/static/css/light.css"
  media="(prefers-color-scheme: light)"
/>

Apply variable as needed:

border: 1px var(--highlights) solid;

You can even update .svg:

svg:not([fill]) {
    fill: var(--foreground);
}

Nice.