add these lines in app/etc/local.xml (or) {yourPKG/yourModule/etc/config.xml} file under <config> tag
Its better If you add it inside your module's Config folder for the reasons specied by @Marius under the comments..
<stores>
<admin>
<design>
<package>
<name>default</name>
</package>
<theme>
<default>MyCustomTheme</default>
</theme>
</design>
</admin>
</stores>
where MyCustomTHeme is your theme folder inside app/design/adminhtml/default/MyCustomTheme
This method takes your MyCustomTheme as default theme and uses FallBack for any layout that is not found.
Adding the above line's inside app/etc/config.xml showed no effect.
And changing the Package folder name from default to "myNewPAckage" had overridden the default and required me to copy whole default into myNewPAckage folder else nothing worked.
So, to have FallBack effect, keep the package as default and give theme as yourCustomTheme.
app/etc/local.xml. Usually this file is not copied between instances (dev, staging, prod) and you might lose the functionality. Use theconfig.xmlfile of a custom module. – Marius Jan 30 '14 at 07:09etc/config.xmltogether, along withapp/etc/local.xmlto a single all inclusive XML configuration.app/etc/local.xmlis loaded first, AND then loaded again last. The reason for this is so that it can receive it's initial configuration values, and so that it also has the last say in configuration (by loading it again always last). – Darren Felton Mar 15 '16 at 19:22