As pheonix says, any changes to a default skin file will be overwritten on an image update.
To keep your changes, it's best to create the custom screen in a simple xml file with only the screens you want modified.
Save it as skin_user_ViX_Night_HD.xml if using the default skin.
Replace ViX_Night_HD in the file name with the skin name if using another skin.
Upload it to /etc/enigma2/ & it should keep working on future updates.
Saying that, you will also lose out on any future updates/improvements made to those screens, if using your own custom one.