For an Ant
build you might have to adjust project.properties; other build systems probably have some other way. And, of course, you need appropriate minSdkVersion and targetSdkVersion values in AndroidManifest.xml.
This answers your second question: http://android-developers.blogspot.de/2012/01/holo-everywhere.html
. If you haven't worked with styles and themes before you should
also read the "Styles and Themes" page linked in there.