Set good default value for cursor theme and size in KRDB

Summary:
BUG: 414109
FIXED-IN: 5.18.2

Reviewers: Plasma, broulik, ervin, meven, crossi, davidedmundson

Reviewed By: Plasma, meven, crossi, davidedmundson

Subscribers: ngraham, davidre, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27470