Public Class CustomActivityDesignerTheme
    Inherits ActivityDesignerTheme
    Public Sub New(ByVal theme As WorkflowTheme)
        MyBase.new(theme)

        MyBase.Initialize()
        Me.BorderStyle = DashStyle.Solid
        Me.BorderColor = Color.FromArgb(0, 0, 0)
        Me.BackColorStart = Color.FromArgb(37, 15, 242)
        Me.BackColorEnd = Color.FromArgb(189, 184, 254)
        Me.BackgroundStyle = LinearGradientMode.Vertical
        Me.ForeColor = Color.Black
    End Sub
End Class

public class CustomActivityDesignerTheme : ActivityDesignerTheme
{
    public CustomActivityDesignerTheme(WorkflowTheme theme)
        : base(theme)
    {
        base.Initialize();
        this.BorderStyle = DashStyle.Solid;
        this.BorderColor = Color.FromArgb(0, 0, 0);
        this.BackColorStart = Color.FromArgb(37, 15, 242);
        this.BackColorEnd = Color.FromArgb(189, 184, 254);
        this.BackgroundStyle = LinearGradientMode.Vertical;
        this.ForeColor = Color.Black;
    }
}

<ActivityDesignerTheme(GetType(CustomActivityDesignerTheme))> _
Public Class CustomActivityDesigner2
    Inherits ActivityDesigner

[ActivityDesignerTheme(typeof(CustomActivityDesignerTheme))]
public class CustomActivityDesigner2 : ActivityDesigner