Classe que aplica tema nos formularios.

Tema.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Drawing;
using System.Windows.Forms;

namespace LogixMine
{
    public class TemaPadrao
    {
        static void AplicaTema(Label l)
        {
            l.Font = new Font("Arial", 11.0f);
            l.BackColor = Color.LightBlue;
            l.ForeColor = Color.Black;
        }

        static void AplicaTema(Form f)
        {
            f.Font = new Font("Arial", 11.0f);
            f.BackColor = Color.LightBlue;
            f.ForeColor = Color.Black;
        }

        static void AplicaTema(Button b)
        {
            if (b.Text.Equals("Salvar"))
            {
                b.BackColor = Color.MediumSeaGreen;
                b.ForeColor = Color.White;
            }
            else if (b.Text.Equals("Cancelar"))
            {
                b.BackColor = Color.IndianRed;
                b.ForeColor = Color.White;
            }
            
        }

        static void AplicaTema(TextBox b)
        {
            b.BorderStyle = BorderStyle.FixedSingle;
        }

        static void UsarTemaGroupBox(GroupBox box)
        {
            foreach (var item in box.Controls)
            {
                if (item.GetType().ToString().Equals("System.Windows.Forms.Label"))
                    AplicaTema((Label)item);

                if (item.GetType().ToString().Equals("System.Windows.Forms.Button"))
                    AplicaTema((Button)item);

                if (item.GetType().ToString().Equals("System.Windows.Forms.TextBox"))
                    AplicaTema((TextBox)item);
            }
        }

        public static void UsarTema(Form formulario)
        {
            AplicaTema(formulario);

            foreach (var obj in formulario.Controls)
            {
                if (obj.GetType().ToString().Equals("System.Windows.Forms.GroupBox"))
                    UsarTemaGroupBox((GroupBox)obj);

                if (obj.GetType().ToString().Equals("System.Windows.Forms.Label"))
                    AplicaTema((Label)obj);

                if (obj.GetType().ToString().Equals("System.Windows.Forms.Button"))
                    AplicaTema((Button)obj);

                if (obj.GetType().ToString().Equals("System.Windows.Forms.TextBox"))
                    AplicaTema((TextBox)obj);
            }            
        }
    }
}