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