using Spire.Doc;
using System.Drawing;

namespace AddBackground
{
    class Program
    {
        static void Main(string[] args)
        {
            //创建一个Document类对象，并加载Word文档
            Document document = new Document();
            document.LoadFromFile(@"C:\Users\Administrator\Desktop\Test.docx");

            //设置文档的背景填充模式为颜色填充
            document.Background.Type = Spire.Doc.Documents.BackgroundType.Color;

            //设置背景颜色
            document.Background.Color = Color.MistyRose;

            //保存并打开文档
            document.SaveToFile("PureBackground.docx", FileFormat.Docx2013);
            System.Diagnostics.Process.Start("PureBackground.docx");
        }
    }
}

using Spire.Doc;
using System.Drawing;
using Spire.Doc.Documents;

namespace AddGradientBackground
{
    class Program
    {
        static void Main(string[] args)
        {
            //创建Document类实例，并加载Word文档
            Document document = new Document();
            document.LoadFromFile(@"C:\Users\Administrator\Desktop\Test.docx");

            //设置文档的背景填充模式为渐变填充
            document.Background.Type = Spire.Doc.Documents.BackgroundType.Gradient;

            //设置渐变背景颜色
            BackgroundGradient gradient = document.Background.Gradient;
            gradient.Color1 = Color.LightSkyBlue;
            gradient.Color2 = Color.PaleGreen;

            //设置渐变模式
            gradient.ShadingVariant = GradientShadingVariant.ShadingMiddle;
            gradient.ShadingStyle = GradientShadingStyle.FromCenter;

            //保存并打开文档
            document.SaveToFile("GradientColor.docx", FileFormat.Docx2013);
            System.Diagnostics.Process.Start("GradientColor.docx");
        }
    }
}

using System.Drawing;
using Spire.Doc;

namespace ImageBackground
{
    class Program
    {
        static void Main(string[] args)
        {
            //创建一个Document类实例，并加载Word文档
            Document document = new Document();
            document.LoadFromFile(@"C:\Users\Administrator\Desktop\Test.docx");

            //设置文档的背景填充模式为图片填充
            document.Background.Type = Spire.Doc.Documents.BackgroundType.Picture;

            //设置背景图片
            document.Background.Picture = Image.FromFile(@"C:\Users\Administrator\Desktop\1.jpg");

            //保存并打开文档
            document.SaveToFile("ImageBackground.docx", FileFormat.Docx2013);
            System.Diagnostics.Process.Start("ImageBackground.docx");
        }
    }
}