How to make a floating ToolWindow transparent?

toolWindow.FindForm().Opacity = .8f;

Form hostForm = toolWindow.FindForm();

hostForm.MouseEnter += new EventHandler(hostForm_MouseEnter);
hostForm.MouseLeave += new EventHandler(hostForm_MouseLeave);

hostForm.Activated += new EventHandler(hostForm_Activated);
hostForm.Deactivate += new EventHandler(hostForm_Deactivate);

void hostForm_Enter(object sender, EventArgs e)
{
    Form f = (Form)sender;
    f.Opacity = 1.0F;
}

void hostForm_Leave(object sender, EventArgs e)
{
    if (_isActivated) return;
    Form f = (Form)sender;
    f.Opacity = 0.80F;
}

private void Form1_Load(object sender, EventArgs e)
        {
            ToolForm f = new ToolForm();
            f.MouseEnter += new EventHandler(f_MouseEnter);
            f.MouseLeave += new EventHandler(f_MouseLeave);
            f.Activated += new EventHandler(f_Activated);
            f.Deactivate += new EventHandler(f_Deactivate);
            f.TopMost = true;
            f.Show();
            
        }

        bool _activated = false;

        void f_Deactivate(object sender, EventArgs e)
        {
            ((Form)sender).Opacity = 0.8F;
            _activated = false;
        }

        void f_Activated(object sender, EventArgs e)
        {
            ((Form)sender).Opacity = 1.0F;
            _activated = true;
        }

        void f_MouseLeave(object sender, EventArgs e)
        {
            if (_activated) return; // if activated, keep the opacity until de-activation
            ((Form)sender).Opacity = 0.8F;
        }

        void f_MouseEnter(object sender, EventArgs e)
        {
            ((Form)sender).Opacity = 1.0F;
        }