I was successful at overriding the /gsdl/web/style/style.css macro in my style.dm file, but was unable to do it in my test collection's extra.dm file. Why? Here is what I put in my test collection's extra.dm file:

# extra.dm file
# You can add collection specific macros in here
# Lines starting with a '#' are comments.
# Remember to include the package declaration

package Style

/gsdl/web/style/style.css[c=test] {/gsdl/web/images/test.css}

- Gavin