Nicely done! Good interpretation of the theme.
The playfield is just about big enough that you can memorize it properly, even for slightly more complicated shapes.
The use of the joker is a nice touch, game-design-wise.
For polish there are things like invert, erase, etc. – It still feels a little bit clunky (what else is new for game-jame games…) as I feel those could have been elements in the interface next to the playfield, but everything you’d want is there already, which is nice.
(Also the background slowly going more black/white to indicate how close you are to the timer is a good subtle hint.)