You can check it out from your own instance by going to https://t.lemmy.world/ (that might be an older version though? at one point Tesseract development was frozen but then it later resumed).

Your instance sidebar also mentions several other alternative UIs:

https://a.lemmy.world/ - Alexandrite UI
https://photon.lemmy.world/ - Photon UI
https://m.lemmy.world/ - Voyager mobile UI
https://old.lemmy.world/ - A familiar UI
https://blorp.lemmy.world/ - Blorp UI