How to customize the CSS in Angular material 7 drag and drop while dragging, drag preview and drag placeholder?

Would something like below accomplish your goal?

Initialize nativeElement x and y

dragStart(e, action) {
    const rect = e.source.element.nativeElement.getBoundingClientRect();

    // initialize start X coord
    this.startX = rect.x;
    // initialize start Y coord
    this.startY = rect.y;
  }

Compare X offset and use rendere2 to set style on nativeElement

dragMoved(e, action) {
    // record new position
    this.currentX = e.event.clientX;
    this.currentY = e.event.clientY;
    // logic to set startX and startY
    // TRYING TO CHANGE CARD BORDER COLOR IF this.endX - this.startX > some number
    if(this.startX < this.currentX){
      this._renderer.setStyle(e.source.element.nativeElement, 'border-style', 'solid');
      this._renderer.setStyle(e.source.element.nativeElement, 'border-color', 'green');
    }
    else if (this.startX > this.currentX){
      this._renderer.setStyle(e.source.element.nativeElement, 'border-style', 'solid');
      this._renderer.setStyle(e.source.element.nativeElement, 'border-color', 'blue');
    }
  }

Revision:

To change color while dragging do the following.

Get reference to #cdkDropList in the view.

@ViewChild('cdkDropList') _dropList:any;

Set index in *ngFor

*ngFor="let action of actions; let i = index"

Pass index to function

(cdkDragMoved)="dragMoved($event, action, i)"

Receive index and reach into the children of the cdkDropList to set the style.

dragMoved(e, action, i) {
    // record new position
    this.currentX = e.event.clientX;
    this.currentY = e.event.clientY;
    // logic to set startX and startY
    // TRYING TO CHANGE CARD BORDER COLOR IF this.endX - this.startX > some number
    if(this.startX < this.currentX){
      this._renderer.setStyle(this._dropList.nativeElement.children[i], 'border-style', 'solid');
      this._renderer.setStyle(this._dropList.nativeElement.children[i], 'border-color', 'green');
    }
    else if (this.startX > this.currentX){
      this._renderer.setStyle(this._dropList.nativeElement.children[i], 'border-style', 'solid');
       this._renderer.setStyle(this._dropList.nativeElement.children[i], 'border-color', 'blue');
    }
  }

Stackblitz

https://stackblitz.com/edit/angular-hdskvc?embed=1&file=app/cdk-drag-drop-overview-example.ts