【解决办法】
// 示例如下,一定是在添加专题图图层后,再给特殊值设置透明风格才会起作用。
ThemeGridUnique themeUnique = new ThemeGridUnique();
themeUnique.add(getThemeItem("1", new Color(0, 0, 255)));
themeUnique.add(getThemeItem("2", new Color(0, 255, 0)));
themeUnique.add(getThemeItem("3", new Color(255, 0, 0)));
Layers layers = map.getLayers();
Layer l = layers.add(datasetGrid, themeUnique, true);
ThemeGridUnique gridUnique1= (ThemeGridUnique) l.getTheme();
gridUnique1.setThemeSpecialValue(0);
gridUnique1.setSpecialValueTransparent(true);