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