how to make android transparent keyboard?

I am writing an Android application for API 2.2, where I need to use transparent keyboard and I have a problem. I can't find how to make keys for the keyboa开发者_运维百科rd. I can set alpha for key icon by setting this.

mQwertyKeyboard.getKeys().get( i ).icon.mutate().setAlpha( 0 );

where mQwertyKeyboard is QWERTYKeyboard extends Keyboard, but how do I set alpha for key background? Thank you for help.