The current pdfsam doesn't honor the swing.defaultlaf, nor does it use swing.installedlafs for populating the list of available lafs in the look and feel selector in settings.
This makes using custom look and feel almost impossible. Currently the only working option how to make pdfsam pick up a custom laf is to set it as a default system laf and select an unsupported laf in pdfsam (eg. the Windows look and feel on linux). PDFsam then prints an error that the laf cannot be used and it is started using the default laf as a fallback.
I think UIManager.getInstalledLookAndFeels() can be used to get the list of installed lafs.