Probably create two classes, one without the trait and one with it and extends the former so that you don't need to copy its code:

class MyReport extends koolreport\KoolReport {
    ...
}
...
class MyReportWithBootstrapCSS extends MyReport { //extends MyReport class to use its settings and setup method
    use  \koolreport\clients\BootstrapCSS;
}
...
if (...) $report = new MyReport();
else if (...) $report = new MyReportWithBootstrapCSS();

Hope this helps.