GTK+ made a change to checkboxes, which makes 'checked' ones not show up properly unless themes update their behaviour. I believe: https://projects.kde.org/projects/playground/artwork/oxygen-gtk/repository/revisions/5f97a2d3f8880eb68ce2e0fd44285b6adc38293e fixes this upstream, and we just need to pull that back downstream. However, this affects the appearance of anaconda during installation from KDE media, so we may want to pull it into F21 Alpha.
oxygen-gtk3-1.4.0-4.fc21 has been submitted as an update for Fedora 21. https://admin.fedoraproject.org/updates/oxygen-gtk3-1.4.0-4.fc21
*** Bug 1133189 has been marked as a duplicate of this bug. ***
*** Bug 1133191 has been marked as a duplicate of this bug. ***
Package oxygen-gtk3-1.4.0-4.fc21: * should fix your issue, * was pushed to the Fedora 21 testing repository, * should be available at your local mirror within two days. Update it with: # su -c 'yum update --enablerepo=updates-testing oxygen-gtk3-1.4.0-4.fc21' as soon as you are able to. Please go to the following url: https://admin.fedoraproject.org/updates/FEDORA-2014-10178/oxygen-gtk3-1.4.0-4.fc21 then log in and leave karma (feedback).
Confirmed this fix by updating oxygen-gtk3 before running liveinst. However it wasn't pulled into the TC6 compose, so there may be more reports of it going forward until the next TC.
Discussed at 2014-09-10 freeze exception review meeting: http://meetbot.fedoraproject.org/fedora-blocker-review/2014-09-10/f21-blocker-review.2014-09-10-16.07.log.txt . Accepted as a freeze exception issue, obviously we want the KDE live installer to not have invisible checkboxes.
oxygen-gtk3-1.4.0-4.fc21 has been pushed to the Fedora 21 stable repository. If problems still persist, please make note of it in this bug report.