Example:

//Instantiating a Workbook object
Workbook workbook = new Workbook();
Cells cells = workbook.getWorksheets().get(0).getCells();
cells.get("A1").putValue("Hello World");
Style style = cells.get("A1").getStyle();
//Set ThemeColorType.Text2 color type with 40% lighten as the font color.
style.getFont().setThemeColor(new ThemeColor(ThemeColorType.TEXT_2, 0.4));
style.setPattern(BackgroundType.SOLID);
//Set ThemeColorType.Background2 color type with 75% darken as the foreground color
style.setForegroundThemeColor(new ThemeColor(ThemeColorType.BACKGROUND_2, -0.75));
cells.get("A1").setStyle(style);
//Saving the Excel file
workbook.save("book1.xlsx");