Where to edit styles if scss and bootstrap are included?

Question

Drupal

I installed a new theme on drupal and it has scss in it, and I can’t figure out the styles, I need to change the weith of the img output for one type of material. Style.scss has
.my-class{
.img{
width: 100%;
}
}
I change it by 20% in this type of material it works, but the template also works on other pages, I try to reach out through other classes and through ID, no way.
and I change it back to 100%, css is not restored, only replacing the style.scss file with the old one from the archive helps. something logic fails me, please tell me what I'm doing wrong.