You can setup a new admin theme, very similar to the way you do it for a frontend theme (add new folders/files in the package and theme locations in app/design/adminhtml and skin/adminhtml). Once that is in place, just override app/code/core/Mage/Adminhtml/etc/config.xml as app/code/local/Myname/Adminhtml/etc/config.xml and add the following code to it:
<!-- default admin design package and theme -->
Also note you will need to enable your new module before the changes are recognized.
Most likely you will want to make a module that represent’s your company to hold all your specific changes. Start off by making a new directory like so:
Now, if you need to make changes to the Magento file, Mage/Catalog/Block/Breadcrumbs.php, you can add a new directory for the “Catalog” module under your XyzCorp directory, then a blocks directory and copy the file into this new directory. Also you need to create config.xml of your module.
Change the class name inside the file by starting off with “XyzCorp” instead of “Mage”.
Strip out all the code you don’t need, and make it subclass the original class name (”Mage_Catalog_Block_Breadcrumbs”).
Now, you must activate your module so Magento theme understands that there is new code in the “local” directory.
In app/etc/modules/XyzCorp_Catalog.xml, add the following lines
It is crucial that the same prefix XyzCorp is used throughout files, class names, directories, and XML tag names.
Now, your own catalog module is activated, but when will it actually be called by the system? Ahh, we need a special rewrite tag to instruct Magento to use this one file (Breadcrumbs.php) instead of its default.
Now we should rewrite block using your module’s config file.