With dark mode selected in macOS Mojave, platform-compliant applications shift their background color slightly to match the user's wallpaper with a feature called "Desktop Tinting". This allows the background color of windows to more closely match the hue of the user's background, and results in the applications and user background appearing more harmonious together than they would otherwise.
For a thorough explanation and example images of this feature, see the "Under the Covers" section of 512 Pixels' Mojave Dark Mode article.
I would absolutely love if Sublime Text also supported this feature. Without it, Sublime text's otherwise green-hued Monokai background clashes with all other windows on the system if a background primarily featuring a color other than green is chosen.