It's not possible to change Kompare preferences.
I tried to change the font size. When I clicked OK or Apply, nothing changed visually.
When I open the Preferences dialog again, the default settings are in place again.

When starting Kompare form the console, those five lines appear when opening the Preferences dialog:

QObject::connect: No such signal KomparePrefDlg::defaultClicked()
QObject::connect: No such signal KomparePrefDlg::helpClicked()
QObject::connect: No such signal KomparePrefDlg::applyClicked()
QObject::connect: No such signal KomparePrefDlg::okClicked()
QObject::connect: No such signal KomparePrefDlg::cancelClicked()

So my guess is that somehow the Preferences dialog's button bindings are broken, so the changed settings are never applied.

Reproducible: Always

Steps to Reproduce:
1. Open Preferences dialog
2. Change something
3. click OK to close the dialog


Actual Results:  
Changed settings are not applied.
When re-opening Preferences, old default settings are still in place.

Expected Results:  
Changed settings are applied.
When re-opening Preferences, the new settings are in place.