图像上的透明手风琴

Question

我的图像上需要透明的手风琴，背景为黑色，文本为白色。我希望手风琴在图像上透明。

我尝试了以下代码：

<form method="post" id="subCommentForm4">
The id of this form is "COMMENTID"
</form>

css代码：

 <div fxLayout="row" fxLayout.xs="column">
          <div class = "promo" fxFlex="100%">
            <img [src]="inspectionDetailedImgUrl" width="100%" alt="Image" />
          </div>
          <div class="hilight" fxFlex.xs="100%">
            <mat-accordion>
              <mat-expansion-panel (opened)="panelOpenState = true" (closed)="panelOpenState = false">
                <mat-expansion-panel-header>
                  <mat-panel-title>
                    Header
                  </mat-panel-title>
                </mat-expansion-panel-header>
               <h1>Welcome</h1>
              </mat-expansion-panel>
            </mat-accordion>
          </div>
  </div>

谢谢。