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The footer.jspf is included by every page and closes off the main page divs, as well as outputting some of its own branded content.
All styles are defined in styles/styles.css with the exception of the body{} override in the modal windows.
Eventually, the default style of SakaiAdminX may change to be more generic and easier to style, but with this information it should be relatively easy to modify the existing structure.
The tag library
The taglibrary wraps up some common functionality into single tags, ie the <e:footer /> tag simply outputs '</body></html>'.
Some other tags also have their own styles applied to them by default. The source code to the taglibrary is readily available and it is all documented.
Images and icons
Some images used by the default SakaiAdminX skin are in images/ and many icons are in images/icons/. A larger set of icons called 'Icojoy are used for most warning/alert/info messages and are in images/icons/icojoy. The main set of icons, Silk, are located at images/icons/silk
You will notice that all references in styles/styles.css are absolute URLs so its possible to locate these icons elsewhere if you desire. You will need to change the reference in the taglibrary Page.java file as well though (see the Installation Guide for more info on configuring the tag library).
Most tag library references to icons use the scheme silk/someicon and the tag library does the rest so as long as the base URL to the iconsets are set correctly, the URLS should resolve.