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The game already makes use of the “night mode” levels, and works with any background color. Here is a demonstration:

This is because the game uses JavaScript forked from the ‘T-Rex runner’ game in the open source project Chromium. In other words, the original Chrome game, and the plugin game come from the same code base:
https://cs.chromium.org/chromium/src/components/neterror/resources/offline.js

The code to set up and run “night mode” is here:
https://plugins.trac.wordpress.org/browser/dinosaur-game/tags/1.0.7/game-logic.js#L2225