It was a term for modded crappy japanese motorcycles and cars, that derived from a slur for asian people.
I think the important bit people miss here is “was”, language changes, it was an insult, now it just means adding shiny stuff to your desktop.
I think the modern usage also has the nuance of fragility and temporality.
You wouldn’t call a polished and extremely stable customisation a ‘rice’, you’d probably call it a theme