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
.