Calc using HTML, CSS Flex Grid, JS

Hi everyone! I haven't been here for a long time. I started with calc. I have begun this project much earlier but I gave up. Today I got myself together and made it done! I know that it's still imperfect but with small steps, we could reach the summit. I would be very thankful for any advice. You could point out each mistake if you wish. I just want to get the things understood. Moreover, I haven't ever written that complicated code in JS, I'm proud of myself. Admittedly, the themes don't work correctly, on my personal computer everything was just fine but on the server, it doesn't work well + if you are trying to perform a particular expression, the calc is going to get stuck.