How can I change the opacity of the task bar?

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http://askubuntu.com – Well .. the question is as simple as the title. But for details .. I really like the look of the task bar when I start ubuntu when I choose from users accounts .. it is simple and has a little transparency .. I want to have these effects on my task bar all the time. How can I change the opacity and the color of the task bar ? Is it themes ?! How can I find trusted ones and how to install them ?! (HowTos)