Fix : Added a media query for dark mode users

[YashNuhash]

Fixes https://github.com/expressjs/expressjs.com/issues/1382

[YashNuhash]

Hey, @dougwilson

Now, you can review my commit

[crandmck]

@chrisdel101 Should this be closed or merged?

[chrisdel101]

This would have handled the system setting, but only changed the main background. So closed since #1490 handles this a bit differently. (ps - reopen if I closed prematurely :) )