Put the whole Alt key in green on keyboard schema

During #18784 (closed), P1 took some time to understand where the Alt key was on the keyboard. In part, she didn't scroll enough and quick enough. But she also blamed her poor sight. Since we're using consistently the green color to denote action (like the Mac animation itself). I think that it can only help to put the Alt key in full green on the keyboard schema as well, for consistency and clarity.