Responsive Theme-able Calculator App Using CSS and Javascript

This is my first Frontend Mentor project, but it was really fun!

• This is my first time using CSS variables and themes. I feel like my variables are not very DRY and could be condensed more. Is there a better way to organize them? Also, I only really used variables for the themes, but if you have any suggestions about ways I could have used it in other areas of the CSS, that would be much appreciated.
• As for the JavaScript, while I went with classes. I feel like some of my methods are still too long; if you could point out some ways to break them into smaller, reusable functions that would be great!
• I bug tested this a lot, but through many code rewrites, I probably missed some things. Let me know if you find any bugs!

I appreciate any and all feedback, including in regards to anything I didn't list above! Thank you :)