You like dragons?
The third way (easier than easy) is to simply set the background colour of the box to a semitransparent value, which is achievable with RGBA colours. Try setting background-color: rgba(0, 0, 0, 0.85) for 85% opaque grey black, for instance.
Disclaimer: be sure to include another background-color attribute before said rule that is a standard hex value, like #000000, to prevent your signature from mangling up in IE8, which still sees some use today. And older feature phones.