I was able to add this checkbox behavior to the darknet-demo-winforms project by making the following changes, along with the fix in 01d809b.

`Form1.cs`

Added a new CheckBox with the following properties

ThreeState = true
CheckState = Indeterminate

Details in Form1.Designer.cs InitializeComponent():

show

  // 
  // darkModeCheckbox
  // 
  this.darkModeCheckbox.Anchor = ((System.Windows.Forms.AnchorStyles)((((System.Windows.Forms.AnchorStyles.Top | System.Windows.Forms.AnchorStyles.Bottom) 
  | System.Windows.Forms.AnchorStyles.Left) 
  | System.Windows.Forms.AnchorStyles.Right)));
  this.darkModeCheckbox.AutoSize = true;
  this.darkModeCheckbox.Checked = true;
  this.darkModeCheckbox.CheckState = System.Windows.Forms.CheckState.Indeterminate;
  this.darkModeCheckbox.Location = new System.Drawing.Point(704, 470);
  this.darkModeCheckbox.Name = "darkModeCheckbox";
  this.darkModeCheckbox.Size = new System.Drawing.Size(148, 29);
  this.darkModeCheckbox.TabIndex = 1;
  this.darkModeCheckbox.Text = "Dark mode";
  this.darkModeCheckbox.ThreeState = true;
  this.darkModeCheckbox.UseVisualStyleBackColor = true;
  this.darkModeCheckbox.CheckStateChanged += new System.EventHandler(this.OnDarkModeCheckboxStateChanged);

Added an event handler for the checkbox's CheckStateChanged event:

private void OnDarkModeCheckboxStateChanged(object sender, EventArgs e) {
    Theme desiredTheme = darkModeCheckbox.CheckState switch {
        CheckState.Checked       => Theme.Dark,
        CheckState.Unchecked     => Theme.Light,
        CheckState.Indeterminate => Theme.Auto,
        _                        => Theme.Auto
    };

    DarkNet.Instance.SetCurrentProcessTheme(desiredTheme);
}