Hi,
That is not default functionality in OJS2. I had this in my old OJS2 theme plugin that enabled me to change the template. I have seen other similar solutions and you could probably find them with a search.
function getStylesheetFilename() {
return 'myTheme.css';
}
function activate(&$templateMgr) {
$theme_template_id = "myTheme";
$theme_template_dir = Core::getBaseDir() . DIRECTORY_SEPARATOR . 'plugins' . DIRECTORY_SEPARATOR . 'themes' . DIRECTORY_SEPARATOR . $theme_template_id . DIRECTORY_SEPARATOR . 'templates';
$app_template_dir = Core::getBaseDir() . DIRECTORY_SEPARATOR . 'templates';
$core_template_dir = Core::getBaseDir() . DIRECTORY_SEPARATOR . 'lib' . DIRECTORY_SEPARATOR . 'pkp' . DIRECTORY_SEPARATOR . 'templates';
$templateMgr->template_dir = array($theme_template_dir, $app_template_dir, $core_template_dir);
$templateMgr->compile_id = $theme_template_id;
if (($stylesheetFilename = $this->getStylesheetFilename()) != null) {
$path = Request::getBaseUrl() . '/' . $this->getPluginPath() . '/' . $stylesheetFilename;
$templateMgr->addStyleSheet($path);
}
}