I searched though Mozilla API, they don't seem to have any variables corresponding to the browser-windows color. Though i found a page that might help you: How to Use Operating System Styles in CSS. Despite the article-header the colors are different for Chrome and Firefox.

This is currently (September 2018) being discussed in "CSS Working Group Editor Drafts". Spec has launched (see above), available as a media query. Something has already landed in Safari, see also here. So in theory you can do this in Safari/Webkit:

@media (prefers-dark-interface) { color: white; background: black }

But it seems that this is private. On MDN a CSS media feature inverted-colors is mentioned. Plug: I blogged about dark mode here.

The new standard is registered on W3C in Media Queries Level 5.

NOTE: currently only available in Safari Technology Preview Release 68

In case user preference is light:

/* Light mode */
@media (prefers-color-scheme: light) {
    body {
        background-color: white;
        color: black;
    }
}

In case user preference is dark:

/* Dark mode */
@media (prefers-color-scheme: dark) {
    body {
        background-color: black;
        color: white;
    }
}

There is also the option no-preference in case a user has no preference. But I recommend you just to use normal CSS in that case and cascade your CSS correctly.

EDIT (7 dec 2018):

In Safari Technology Preview Release 71 they announced a toggle switch in Safari to make testing easier. I also made a test page to see the browser behaviour.

If you have Safari Technology Preview Release 71 installed you can activate through:

Develop > Experimental Features > Dark Mode CSS Support

Then if you open the test page and open the element inspector you have a new icon to toggle Dark/Light mode.

EDIT (11 feb 2019): Apple ships in the new Safari 12.1 dark mode

EDIT (5 sep 2019): Currently 25% of the world can use dark mode CSS. Source: caniuse.com

Upcoming browsers:

• iOS 13 ( I guess it will be shipped next week after Apple's Keynote)
• EdgeHTML 76 (not sure when that will be shipped)

EDIT (5 nov 2019): Currently 74% of the world can use dark mode CSS. Source: caniuse.com

EDIT (3 Feb 2020): Microsoft Edge 79 supports dark mode. (released on 15 Jan 2020)

My suggestion would be: that you should consider implementing dark mode because most of the users can use it now (for night-time users of your site).

Note: All major browsers are supporting dark mode now, except: IE, Edge

EDIT (19 Nov 2020): Currently 88% of the world can use dark mode CSS. Source: caniuse.com

CSS-framework Tailwind CSS v2.0 supports dark-mode. (released on 18 Nov 2020)

EDIT (2 Dec 2020):

Google Chrome adds Dark Theme emulation to Dev Tools. Source: developer.chrome.com

If you want to detect it from JS, you can use this code:

if (window.matchMedia && window.matchMedia('(prefers-color-scheme: dark)').matches) {
    // dark mode
}

To watch for changes:

window.matchMedia('(prefers-color-scheme: dark)').addEventListener('change', e => {
    const newColorScheme = e.matches ? "dark" : "light";
});

According to Mozilla, here is the preferred method as of 2020

@media (prefers-color-scheme: dark) {
  body {
    background: #000;
  }
}
@media (prefers-color-scheme: light) {
  body {
    background: #fff;
  }
}

For Safari/Webkit you can use

@media (prefers-dark-interface) { background: #000; }