I guess you have seen the new Android design guidelines, with the new \"Floating Action Button\" a.k.a \"FAB\"
For instance this pink button:
Try this library (javadoc is here), min API level is 7:
dependencies {
compile 'com.shamanland:fab:0.0.8'
}
It provides single widget with ability to customize it via Theme, xml or java-code.
It's very simple to use. There are available normal and mini implementation according to Promoted Actions pattern.
Try to compile the demo app. There is exhaustive example: light and dark themes, using with ListView, align between two Views.