Form1.cs

using System.Diagnostics.Contracts;
using System.Runtime.InteropServices;

namespace ClipLab
{
    public partial class Form1 : Form
    {
        //Fields
        private Button currentButton;
        private Random random;
        private int tempIndex;
        private Form activeForm;

        //Constructor 
        public Form1()
        {
            InitializeComponent();
            

            random = new Random();
            btnCloseChildForm.Visible = false;
            this.Text = string.Empty;
            this.ControlBox = false;

            this.MaximizedBounds = Screen.FromHandle(this.Handle).WorkingArea;

        }

        [DllImport("user32.DLL", EntryPoint = "ReleaseCapture")]
        private extern static void ReleaseCapture();
        [DllImport("user32.DLL", EntryPoint = "SendMessage")]
        private extern static void SendMessage(System.IntPtr hWnd, int wMsg, int wParam, int lParam);

        //Methods
        private Color SelectThemeColor()
        {
            int index = random.Next(ThemeColor.ColorList.Count);
            while (tempIndex == index) {
              index= random.Next(ThemeColor.ColorList.Count);
            }
            tempIndex = index;
            string color = ThemeColor.ColorList[index];
            return ColorTranslator.FromHtml(color);
        }

        private void ActivateButton(object btnSender)
        {
            if (btnSender != null)
            {
                if (currentButton != (Button)btnSender)
                {
                    DisableButton();
                    Color color = SelectThemeColor();
                    currentButton = (Button)btnSender;
                    currentButton.BackColor = color;
                    currentButton.ForeColor = Color.White;
                    currentButton.Font = new System.Drawing.Font("Segoe UI", 12.5F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point);
                    panelTitleBar.BackColor = color;
                    panelLogo.BackColor = ThemeColor.ChangeColorBrightness(color, -0.3);
                    ThemeColor.PrimaryColor = color;
                    ThemeColor.SecondaryColor = ThemeColor.ChangeColorBrightness(color, -0.3);
                    btnCloseChildForm.Visible = true;

                }
            }
        }
        private void DisableButton()
        {
            foreach(Control previousBtn in panelMenu.Controls)
            {
                if (previousBtn.GetType() == typeof(Button))
                {
                    previousBtn.BackColor = Color.FromArgb(41, 45, 62);
                    previousBtn.ForeColor = Color.Gainsboro;
                    previousBtn.Font = new System.Drawing.Font("Segoe UI", 10F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point);
                }
            }
        }

        private void OpenChildForm(Form childForm, object btnSender)
        {
            if (activeForm != null)
            {
                activeForm.Close();
            }
            ActivateButton(btnSender);
            activeForm = childForm;
            childForm.TopLevel = false;
            childForm.FormBorderStyle = FormBorderStyle.None;
            childForm.Dock = DockStyle.Fill;
            this.panelDesktopPanel.Controls.Add(childForm);
            this.panelDesktopPanel.Tag = childForm;
            childForm.BringToFront();
            childForm.Show();
            lblTitle.Text = childForm.Text;
        }
        private void LblHintDisable()
        {
            lblHint.Visible = false;
        }
        private void btnDownload_Click(object sender, EventArgs e)
        {
            OpenChildForm(new Forms.FormDownload(),sender);
            LblHintDisable();
        }

        private void btnEdit_Click(object sender, EventArgs e)
        {
            OpenChildForm(new Forms.FormEdit(), sender);
            LblHintDisable();

        }

        private void btnInfo_Click(object sender, EventArgs e)
        {
            // new OpenChildForm(new Forms.FormInfo(), sender);

            OpenChildForm(new Forms.FormInfo(), sender);
            LblHintDisable();

        }

        private void btnCloseChildForm_Click(object sender, EventArgs e)
        {
            if (activeForm != null)
                activeForm.Close();
            Reset();
        }

        private void Reset()
        {
            DisableButton();
            lblTitle.Text = "ÃÎËÎÂÍÀ";
            panelTitleBar.BackColor = Color.FromArgb(0, 133, 137);
            panelLogo.BackColor = Color.FromArgb(39, 38, 60);
            currentButton = null;
            btnCloseChildForm.Visible = false;
        }

        private void panelTitleBar_MouseDown(object sender, MouseEventArgs e)
        {
            ReleaseCapture();
            SendMessage(this.Handle, 0x112, 0xf012, 0);
        }

        private void btnClose_Click(object sender, EventArgs e)
        {
            Application.Exit();
        }

        private void btnMaximize_Click(object sender, EventArgs e)
        {
            if(WindowState==FormWindowState.Normal)
                this.WindowState = FormWindowState.Maximized;
            else
                this.WindowState = FormWindowState.Normal;
        }

        private void btnMinimize_Click(object sender, EventArgs e)
        {
            this.WindowState = FormWindowState.Minimized;
        }
    }
}