There's a global setting in Dark Reader to "Detect dark theme" pages -- when that is enabled, this game is automatically detected as using a dark theme so the Dark Reader plugin is disabled for the page.
But the Clone-a-Lisa page does not use a native dark theme, and the problem seem to be just with the paint colors, which in my case for some reason are altered by Dark Reader. It could also be that it was a temporarily glitch as I've noticed that color manipulation by Dark Reader can be inconsistent at times (could be due to DOM loading timing and stuff like that).
There's a global setting in Dark Reader to "Detect dark theme" pages -- when that is enabled, this game is automatically detected as using a dark theme so the Dark Reader plugin is disabled for the page.