Make the dialogs consistent with others KCMs by using QtQuick versions.
Test dialogs for GTK themes installation.
No I see it as a consistency/polish/bugfix thing. The rest of the KCMs got ported to use the new dialog for Plasma 5.18; it would be a bit weird if this one didn't get the same treatment.
Looks good, and this should go on the plasma/5.18 branch to make sure it gets into Plasma 5.18 so this KCM isn't the odd one out.
However it contains new and changed strings so I have requested a string freeze exception to make this possible. I'll let you know if and when it's accepted so this patch can land. Until then, please do not land it.