ThemeManager.cs

using System;
using System.Drawing;
using System.Windows.Forms;
using Microsoft.Win32;

namespace Yotepad
{
    public class ThemeManager
    {
        public bool IsDarkMode { get; private set; }

        public Color BackgroundColor => IsDarkMode ? Color.FromArgb(30, 30, 30) : Color.White;
        public Color TextColor => IsDarkMode ? Color.FromArgb(220, 220, 220) : Color.Black;
        public Color MenuBackgroundColor => IsDarkMode ? Color.FromArgb(45, 45, 45) : SystemColors.Control;

        public void InitializeTheme()
        {
            try
            {
                using (RegistryKey? key = Registry.CurrentUser.OpenSubKey(@"Software\Microsoft\Windows\CurrentVersion\Themes\Personalize"))
                {
                    if (key?.GetValue("AppsUseLightTheme") is int lightTheme)
                    {
                        IsDarkMode = (lightTheme == 0);
                    }
                }
            }
            catch { IsDarkMode = true; }
        }

        public void ToggleTheme() => IsDarkMode = !IsDarkMode;

        public void ApplyTheme(Form form, TextBox textBox, MenuStrip menu, StatusStrip status)
        {
            // Call the undocumented API: 2 = ForceDark, 0 = Default Light
            try { NativeMethods.SetPreferredAppMode(IsDarkMode ? 2 : 0); } catch { }

            int darkVal = IsDarkMode ? 1 : 0;
            NativeMethods.DwmSetWindowAttribute(form.Handle, NativeMethods.DWMWA_USE_IMMERSIVE_DARK_MODE, ref darkVal, sizeof(int));

            NativeMethods.SetWindowTheme(textBox.Handle, IsDarkMode ? "DarkMode_Explorer" : "Explorer", null);

            form.BackColor = BackgroundColor;
            textBox.BackColor = BackgroundColor;
            textBox.ForeColor = TextColor;

            menu.Renderer = IsDarkMode ? new YotePadMenuRenderer(this) : new ToolStripProfessionalRenderer();
            menu.BackColor = MenuBackgroundColor;
            menu.ForeColor = TextColor;

            status.BackColor = MenuBackgroundColor;
            status.ForeColor = TextColor;

            foreach (ToolStripMenuItem item in menu.Items)
            {
                ApplyMenuTheme(item);
            }
        }

        private void ApplyMenuTheme(ToolStripMenuItem item)
        {
            item.ForeColor = TextColor;
            if (item.DropDown is ToolStripDropDownMenu dropDown)
            {
                dropDown.ShowImageMargin = false;
                dropDown.BackColor = MenuBackgroundColor;
                dropDown.ForeColor = TextColor;
            }

            foreach (ToolStripItem subItem in item.DropDownItems)
            {
                if (subItem is ToolStripMenuItem subMenu)
                {
                    subMenu.BackColor = MenuBackgroundColor;
                    ApplyMenuTheme(subMenu);
                }
            }
        }
    }
    public class YotePadMenuRenderer : ToolStripProfessionalRenderer
{
    private readonly ThemeManager _theme;

    public YotePadMenuRenderer(ThemeManager theme) : base()
    {
        _theme = theme;
    }

    protected override void OnRenderMenuItemBackground(ToolStripItemRenderEventArgs e)
    {
        if (!_theme.IsDarkMode)
        {
            base.OnRenderMenuItemBackground(e);
            return;
        }

        Rectangle rect = new Rectangle(Point.Empty, e.Item.Size);

        if (e.Item.Selected || e.Item.Pressed)
        {
            using (SolidBrush brush = new SolidBrush(Color.FromArgb(80, 80, 80)))
            {
                e.Graphics.FillRectangle(brush, rect);
            }
        }
        else
        {
            using (SolidBrush brush = new SolidBrush(_theme.MenuBackgroundColor))
            {
                e.Graphics.FillRectangle(brush, rect);
            }
        }
    }

    protected override void OnRenderToolStripBackground(ToolStripRenderEventArgs e)
    {
        if (!_theme.IsDarkMode)
        {
            base.OnRenderToolStripBackground(e);
            return;
        }

        using (SolidBrush brush = new SolidBrush(_theme.MenuBackgroundColor))
        {
            e.Graphics.FillRectangle(brush, e.AffectedBounds);
        }
    }

    protected override void OnRenderSeparator(ToolStripSeparatorRenderEventArgs e)
    {
        if (!_theme.IsDarkMode)
        {
            base.OnRenderSeparator(e);
            return;
        }

        int y = e.Item.Height / 2;
        using (Pen pen = new Pen(Color.FromArgb(70, 70, 70)))
        {
            e.Graphics.DrawLine(pen, 4, y, e.Item.Width - 4, y);
        }
    }

    protected override void OnRenderItemText(ToolStripItemTextRenderEventArgs e)
    {
        if (!_theme.IsDarkMode)
        {
            base.OnRenderItemText(e);
            return;
        }

        e.TextColor = e.Item.Enabled 
            ? Color.FromArgb(220, 220, 220) 
            : Color.FromArgb(110, 110, 110);
        base.OnRenderItemText(e);
    }
}
}