This is a split-off of https://phabricator.kde.org/D5111 , proposing the addition of a colour scheme chooser to the Oxygen demo application.
Screenshots:
https://phabricator.kde.org/file/data/3p3ojesf6x6u3psaldn7/PHID-FILE-hwpoh2rgdam6fbzyawel/oxydemo-qtcurve-menu.png
https://phabricator.kde.org/file/data/bi7rwuxqa3dxpehemhxg/PHID-FILE-33avytvqsdtnuygaywso/oxydemo-oxygen-menu.png