[WIP][API Suggestion] Introduce Win11 Dark Mode to Forms and default application settings

Rationale

With a few new APIs which the Windows Team now made publicly available, we can start to introduce DarkMode functionality for WinForms. This is one of our most requested customer features. By introducing CsWin32 to our WinForms repo, adapting more and more Windows DarkMode Features overtime to WinForms is also comparatively easy to achieve.

This API proposal starts with a general concept for how to handle DarkMode in WinForms, and how to apply DarkMode look and feel just to a WinForms Form title bar.

List of new APIs

New DarkMode enum

namespace System.Windows.Forms
{
    public enum DarkMode
    {
        /// <summary>
        ///  Dark mode in the current context is not supported.
        /// </summary>
        NotSupported = 0,

        /// <summary>
        /// The setting for the current dark mode context is inherited from the parent context.
        /// </summary>
        Inherits = 1,

        /// <summary>
        /// Dark mode for the current context is enabled.
        /// </summary>
        Enabled = 2,

        /// <summary>
        /// Dark mode the current context is disabled.
        /// </summary>
        Disabled = 3
    }
}

Changed to the Application class

namespace System.Windows.Forms
{
    public sealed partial class Application
    {
            public static DarkMode DefaultDarkMode { get; } // Gets the default dark mode.
            public static DarkMode EnvironmentDarkMode { get;} // Gets if the dark mode is set for the OS or if it is not supported.

            public static bool SetDefaultDarkMode(DarkMode darkMode); // Sets the default dark mode and returns if it could be set or not.
    }
}

Changes to the Controls class

namespace System.Windows.Forms
{
    public partial class Control :
        Component,
        Ole32.IOleControl,
              .
              .
              .
    {
        public DarkMode DarkMode {get; set;} // Sets or gets the dark mode for this control. This property is ambient. The setter calls SetDarkModeCore, which can be overwritten in inherited classes
        protected virtual bool IsDarkModeEnabled {get;} // Gets a value indicating if the dark mode is actually enabled for this control
        protected virtual void SetDarkModeCore(DarkMode darkModeSetting); // The core to change the dark mode for the control. Is virtual, so an inherited class can add necessary implementations if necessary. Called by the setter of the DarkMode property.
    }
}

Changes to the Forms class

namespace System.Windows.Forms
{
    public partial class Form : ContainerControl
    {
        public Color WindowBorderColor { get; set; } // Sets or gets the Window's border color.
        public int WindowBorderThickness { get; set; } // Sets or gets the Window's border thickness.
        public Color WindowCaptionColor { get; set; } // Sets or gets the Window's caption color.
        public Color WindowCaptionTextColor { get; set; } // Sets or gets the Window's caption text color.
    }
}

[driver1998]

Is the dark common control theme documented now? Last time I checked it is still undocumented. Only the dark title bar attribute is documented and public.

[Balkoth]

Dark mode is easily my most wanted feature for winforms. Second would be a working PMv2 mode…