Calculator app with SASS and vanilla JS

I learned to use desktop theme preference. And to change theme between 3 themes. For get the numbers to calculate I used e.target.id of each buttons. When we click on an operator button (+, -, /,*) the number (e.target.id) became a "previousNumber" and be display in "previousdisplay" this number will be calculate with the next numbers until we click on egal for have the total. Every time we click on a operator button we will get preview of the total price in "previousdisplay"