C# 中的自定义按钮：如何删除悬停背景？

我正在尝试使用 Visual Studio 2005 对我的表单（其 FormBorderStyle = none）执行自定义按钮。我在链接到该按钮的 ImageList 中有我的 3 种状态按钮图像。

this.btnClose.AutoSize = false;
this.btnClose.BackColor = System.Drawing.Color.Transparent;
this.btnClose.BackgroundImageLayout = System.Windows.Forms.ImageLayout.Center;
this.btnClose.FlatAppearance.BorderSize = 0;
this.btnClose.FlatStyle = System.Windows.Forms.FlatStyle.Flat;
this.btnClose.ForeColor = System.Drawing.Color.Transparent;
this.btnClose.ImageKey = "Disabled";
this.btnClose.ImageList = this.imageList1;
this.btnClose.Location = new System.Drawing.Point(368, -5);
this.btnClose.Margin = new System.Windows.Forms.Padding(0);
this.btnClose.Name = "btnClose";
this.btnClose.Size = new System.Drawing.Size(31, 31);
this.btnClose.TabIndex = 0;
this.btnClose.UseVisualStyleBackColor = false;
this.btnClose.MouseLeave += new System.EventHandler(this.btnClose_MouseLeave);
this.btnClose.Click += new System.EventHandler(this.btnClose_Click);
this.btnClose.MouseDown += new System.Windows.Forms.MouseEventHandler(this.btnClose_MouseDown);
this.btnClose.MouseHover += new System.EventHandler(this.btnClose_MouseHover);

private void btnClose_MouseHover(object sender, EventArgs e)
{
    btnClose.ImageKey = "enabled";
}

private void btnClose_MouseDown(object sender, MouseEventArgs e)
{
    btnClose.ImageKey = "down";
}

private void btnClose_MouseLeave(object sender, EventArgs e)
{
    btnClose.ImageKey = "disabled";
}

一切正常，但有一个问题。每当我移动鼠标悬停按钮时，我都会看到一个非常烦人的灰色背景。

我怎样才能删除它？

btnClose.FlatAppearance.MouseOverBackColor = System.Drawing.Color.Transparent;