This is a minimal sample to test the dark mode compatibility on web in iOS13 and MacOS Mojave.
As of now, It only works in safari; you can see a Demo here.
I'm using :root and prefers-color-scheme: to specify the darkmode style. If you're not comfortable with the css variables you can also use selectors.
Read also: Apple WWDC 2019 | Supporting Dark Mode in Your Web Content