Isn’t just a theme, is a completely different application built on top of Invidious’ API. What works completely differently to Invidious’ current UI. A LOT of things are handled different then how Invidious handles it on their frontend. Replacing Invidious current interface with Materialious isn’t a good idea, because its quite a bit bigger & requires JS compared to Invidious’ current UI.
Calling Materialious a theme would be like calling clipious a theme.