We will begin by copying the custom.css file under skin/adminhtml/default/default/ into the new theme folder, bookstore, under skin/adminhtml/default/bookstore/. In fact, if you take a look at the main.xml file located under app/design/adminhtml/default/default/layout/, you can see that there are some CSS files declared, and one of those is custom.css. The content of the custom.css file is as follows:

<action method="addCss">

<name>custom.css</name>

</action>

Now, to run a quick test and check whether the module is working and if the fallback loads our file correctly, insert the following code inside custom.css to color the header background in black:

.header { background: #000; }

Now save the file. If everything is working, you will...