您在以下方面做错了：

1-您没有设置任何背景颜色；

2-当您创建新单元格时，它们会覆盖行样式，因此您需要为您创建的每个新单元格设置样式；

以下是工作代码：

FileOutputStream fileOut = new FileOutputStream("poi-test.xls");
HSSFWorkbook workbook = new HSSFWorkbook();
HSSFSheet sheet = workbook.createSheet("Readings");
CellStyle style = workbook.createCellStyle();
Font font = workbook.createFont();
font.setFontHeightInPoints((short)11);
font.setFontName(HSSFFont.FONT_ARIAL);
font.setBoldweight(HSSFFont.COLOR_NORMAL);
font.setBold(true);
font.setColor(HSSFColor.DARK_BLUE.index);

style.setFont(font);
//Add these lines     
style.setFillForegroundColor(IndexedColors.AQUA.getIndex());
style.setFillPattern(CellStyle.SOLID_FOREGROUND);

sheet.createFreezePane(0, 1); // Freeze 1st Row   sheet.createFreezePane(int colSplit, int rowSplit, int leftmostColumn, int topRow)


 HSSFRow rowhead = sheet.createRow((short) 0);
 rowhead.setRowStyle(style);
 //Set the cell0 Style        
 HSSFCell cell0 = rowhead.createCell(0);
 cell0.setCellStyle(style);
 cell0.setCellValue("ROW");
 //Set the cell1 Style        
 HSSFCell cell1 = rowhead.createCell(1);
 cell1.setCellStyle(style);
 cell1.setCellValue("NUMBER");

 workbook.write(fileOut);

文件输出：

首先删除font. setBold权重（HSSFFont.COLOR_NORMAL）；。COLOR_NORMAL不是真正的Bold权重，而是一种颜色。font.setBold（true）；应该足以加粗您的文本。

您还需要在样式中添加一些属性以获得背景颜色。这是令人困惑的部分。方法是使用填充。填充的前景和背景颜色与单元格的其余部分不同。要获得实心填充，您需要使用：

style.setForegroundColor(IndexedColors.LIGHT_BLUE.getIndex());
style.setFillPattern(CellStyle.SOLID_FOREGROUND);

请注意，这将填充的前景色设置为浅蓝色。由于填充具有实心前景，这就是您所需要的。如果您选择了其中一种模式填充，则还需要设置填充的背景色。填充本身就是单元格背景。

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

import org.apache.poi.ss.usermodel.Cell;
import org.apache.poi.ss.usermodel.CellStyle;
import org.apache.poi.ss.usermodel.IndexedColors;
import org.apache.poi.ss.usermodel.Row;
import org.apache.poi.xssf.usermodel.XSSFFont;
import org.apache.poi.xssf.usermodel.XSSFSheet;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;

public class excelStylingExample {

    public static void main(String[] args) throws FileNotFoundException, IOException {
        String toCreateFullPath="C:\\Users\\Your Path\\testExcel.xlsx";
        Path path=Paths.get(toCreateFullPath);
        if(!Files.exists(path.getParent())){

            try {
                Files.createDirectory(path.getParent());
                System.out.println("Directory created");
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            System.out.println("Error Directory is Not Created");
        }
    }

    XSSFWorkbook workbook =new XSSFWorkbook();
    XSSFSheet sheet = workbook.createSheet("testSheet");
    String[] columns = {"Code", "Name", "Address" };
    CellStyle hStyle=null;
    // Creating a font
    XSSFFont font= workbook.createFont();
    font.setFontHeightInPoints((short)12);
    font.setFontName("Arial");
    font.setColor(IndexedColors.YELLOW.getIndex());
    font.setBold(true);
    font.setItalic(false);

    hStyle=workbook.createCellStyle();
    hStyle.setFillForegroundColor(IndexedColors.GREEN.getIndex());
    hStyle.setFillPattern(CellStyle.SOLID_FOREGROUND);
    hStyle.setAlignment(CellStyle.ALIGN_CENTER);
    // Setting font to style
    hStyle.setFont(font);


    // Create a Row
    Row headerRow = sheet.createRow(0);
    // Create cells
    for(int i = 0; i < columns.length; i++) {
        Cell cell = headerRow.createCell(i);
        cell.setCellValue(columns[i]);
        // Setting cell style
        cell.setCellStyle(hStyle);  
        sheet.autoSizeColumn(i);
    }
    
    // setting values into records
    int rowCount = 0;
    for(int i=0;i<1;i++) { // an example of one row 
        
        CellStyle styleAllCell=null;
        styleAllCell=workbook.createCellStyle();
        styleAllCell.setAlignment(CellStyle.ALIGN_CENTER);
        
        Row row = sheet.createRow(++rowCount);
        Cell cell = row.createCell(0);
        cell.setCellValue("1000");
        sheet.autoSizeColumn(0);
        cell.setCellStyle(styleAllCell);
        
        cell = row.createCell(1);
        cell.setCellValue("Leaonardo");
        sheet.autoSizeColumn(1);
        cell.setCellStyle(styleAllCell);

        
        cell = row.createCell(2);
        cell.setCellValue("Italy");
        sheet.autoSizeColumn(2);
        cell.setCellStyle(styleAllCell);

    }

    try (FileOutputStream outputStream = new FileOutputStream(toCreateFullPath)) {

        workbook.write(outputStream);

    }


}

}