it should match your theme exactly.
the shout button draws its code from the default css file
background: url("theme/images/theme/submit_bg.png") 0px -140px no-repeat
it uses the same button setup that all the other buttons in your forum use (the post buttons, preview, reply buttons etc)
you could recode it to use a totally different button but that would involve some coding. What theme are you using :S it may just be that the theme doesnt a very good submit button.
two ways of doing it.
changing that button for your entire site, so all the other buttons like save, preview etc changed, or recoding the shout box to use a totally different button configuration. the former is easier, would involve changing the default index.css and the button image.