open system settings > "regional settings" > "Date and time". See the screenshot.
Created attachment 109921 [details] screenshot
This is KDatePicker; reassigning.
Regression from porting to KF5. KWidgetsAddons can no longer use KColorScheme, so it uses Qt::darkRed hardcoded color. It should use a lighter red if the background is dark. See https://cgit.kde.org/kwidgetsaddons.git/tree/src/kdatetable.cpp#n282 and https://cgit.kde.org/kwidgetsaddons.git/tree/src/kdatetable.cpp#n369
Patch available: https://phabricator.kde.org/D12756
*** Bug 447794 has been marked as a duplicate of this bug. ***