you know the Fast and the Furious movies? at least the original 2 or 3, those cars were all tricked out with neon lights, decals, nitros, custom exhaust, all that? most of those cars were Japanese cars that were heavily modified. Basically it was a derogetory term for modifying a piece of shit car to look good, Especally if it was a Japanese car. you slap a body kit on it, neon lights, slap in some bucket seats, switch out the exhaust, but you dont' touch the engine. that's a "Ricer" it's not a good thing in that specific car culture.

So for whatever reason someone at some point was modifying their Desktop Environment or Window Manager with neon borders and all that and decided to call it a Rice. You're essentially modifying your OS without touching the "engine" so to speak. You're just slapping a body kit, neon lights, some bucket seats etc onto your operating system.