Hi Eiji,
Thank you for your kind suggestions! I will look into 1 and 5. I’m hoping to improve my skills so I can use them to make the game better (looking).
Funny you mention 2048 because one of my earlier versions was made with 2048-like styling! In the end I didn’t want to copy their styling too much…
Your point 3: I hope children will like and play my games and improve their math skills without them knowing they do 
I immediately changed the code so the light/dark mode is persistent now! If you do a hard refresh you will see the result.
Again: thank you so much for your reply and great suggestions!