1. Some roms don't have weather theme pre-installed.2. If you have it already. Check from Settings->Apps->All->Weather Theme if you can unistall it.3. If first or second options won't work then root is your only way to do this.Delete LGWeatherTheme.apk and odex file from your system/app folder.Reboot your phone.After that install this and Flex theme from MultiHome Themes.