How to use the @fullcalendar/core.applyStyle function in @fullcalendar/core

doRevertAnimation(callback: () => void, revertDuration: number) {
    let mirrorEl = this.mirrorEl!
    let finalSourceElRect = this.sourceEl!.getBoundingClientRect() // because autoscrolling might have happened

    mirrorEl.style.transition =
      'top ' + revertDuration + 'ms,' +
      'left ' + revertDuration + 'ms'

    applyStyle(mirrorEl, {
      left: finalSourceElRect.left,
      top: finalSourceElRect.top
    })

    whenTransitionDone(mirrorEl, () => {
      mirrorEl.style.transition = ''
      callback()
    })
  }

assignSegCss(segs: Seg[]) {

    for (let seg of segs) {
      applyStyle(seg.el, this.generateSegCss(seg))

      if (seg.level > 0) {
        seg.el.classList.add('fc-time-grid-event-inset')
      }

      // if the event is short that the title will be cut off,
      // attach a className that condenses the title into the time area.
      if (seg.eventRange.def.title && seg.bottom - seg.top < 30) {
        seg.el.classList.add('fc-short') // TODO: "condensed" is a better name
      }
    }
  }

top = options.top || 0;
        if (options.left !== undefined) {
            left = options.left;
        }
        else if (options.right !== undefined) {
            left = options.right - elDims.width; // derive the left value from the right value
        }
        else {
            left = 0;
        }
        // constrain to the view port. if constrained by two edges, give precedence to top/left
        top = Math.min(top, clippingRect.bottom - elDims.height - this.margin);
        top = Math.max(top, clippingRect.top + this.margin);
        left = Math.min(left, clippingRect.right - elDims.width - this.margin);
        left = Math.max(left, clippingRect.left + this.margin);
        applyStyle(el, {
            top: top - origin.top,
            left: left - origin.left
        });
    };
    // Triggers a callback. Calls a function in the option hash of the same name.

getMirrorEl(): HTMLElement {
    let sourceElRect = this.sourceElRect!
    let mirrorEl = this.mirrorEl

    if (!mirrorEl) {
      mirrorEl = this.mirrorEl = this.sourceEl!.cloneNode(true) as HTMLElement // cloneChildren=true

      // we don't want long taps or any mouse interaction causing selection/menus.
      // would use preventSelection(), but that prevents selectstart, causing problems.
      mirrorEl.classList.add('fc-unselectable')

      mirrorEl.classList.add('fc-dragging')

      applyStyle(mirrorEl, {
        position: 'fixed',
        zIndex: this.zIndex,
        visibility: '', // in case original element was hidden by the drag effect
        boxSizing: 'border-box', // for easy width/height
        width: sourceElRect.right - sourceElRect.left, // explicit height in case there was a 'right' value
        height: sourceElRect.bottom - sourceElRect.top, // explicit width in case there was a 'bottom' value
        right: 'auto', // erase and set width instead
        bottom: 'auto', // erase and set height instead
        margin: 0
      })

      this.parentNode.appendChild(mirrorEl)
    }

    return mirrorEl
  }

top = options.top || 0
    if (options.left !== undefined) {
      left = options.left
    } else if (options.right !== undefined) {
      left = options.right - elDims.width // derive the left value from the right value
    } else {
      left = 0
    }

    // constrain to the view port. if constrained by two edges, give precedence to top/left
    top = Math.min(top, clippingRect.bottom - elDims.height - this.margin)
    top = Math.max(top, clippingRect.top + this.margin)
    left = Math.min(left, clippingRect.right - elDims.width - this.margin)
    left = Math.max(left, clippingRect.left + this.margin)

    applyStyle(el, {
      top: top - origin.top,
      left: left - origin.left
    })
  }