A calculator app with react

I've had a hard time with the theme part since I'm new with react and I found 3-state toggle is much more difficult than 2-state, I'd love if there are any tips on how to do it because I plan to come back to this project later for improvements... Note: The decimal point works it's just that I used toLocalString to format the numbers, if there is another way to do it please let me know

And as always, any other feedback is very much appreciated.