This is really nice, taking cues from Undertale works really well in this context. Colours & animations are all great, and I love the dot-matrix priter-ey sound.
I’d like to be able to play the last move - I know it technically doesn’t matter, but I wanted the whole board filled in. More indication of a win would be good - say, if the winning line were highlighted in some way (or all the other squares faded?). Some sound at the start and some sound that indicates win/lose/draw would make it pop a bit more. Making the button match the the rest of the UI a bit more would help also (the lighter font and borders jar a bit with the board and the text at the top, and the highlight colour could do with matching either the yellow or the white) A really nice thing though, really effective.