I updated the Wiki after more investigation.
- Backup / remove the old ksplashrc file
mv ~/.config/ksplashrc ~/.config/ksplashrc.bak- Go to System Settings
- Type in
Splash - Click on Splash Screen
- Select and Apply your desired splash screen
This way you don’t need to completely undo any custom themes, icons, colors, etc, just to bring back a working “Apply” button. 
Here is a comparison between the old (broken) and newly created ksplashrc config files:
Old (renamed to ksplashrc.bak):
[KSplash]
Theme=org.kde.breath2.desktop
New (automatically created upon applying your desired splash screen when no available ksplashrc file currently exists):
[KSplash]
Theme=org.manjaro.breath2.desktop