per-progress {
      --paper-progress-secondary-color: #f8bbd0;
    }

To change the progress bar background color:

    paper-progress {
      --paper-progress-container-color: #64ffda;
    }

Add the class `transiting` to a paper-progress to animate the progress bar when
the value changed. You can also customize the transition:

    paper-progress {
      --paper-progress-transition-duration: 0.08s;
      --paper-progress-transition-timing-function: ease;
      --paper-progress-transition-transition-delay