Yes, I have also cleared cache but no luck.

I was able to change all button color using below code but I dont want to apply class by class.

.bx-btn.bx-btn-primary,bx-btn {

  background-color: #8a00ba;

  background-image: linear-gradient(90deg, #8a00ba 0, #bf08ff 50%, #8a00ba 100%);

  box-shadow: 0 1px 2px 0 rgba(138, 0, 186, 0.5);

  color: #fff;

}