put this in your theme.skin file:
<asp:CreateUserWizard runat="server"
CancelButtonStyle-CssClass="art-button"
ContinueButtonStyle-CssClass="art-button"
CreateUserButtonStyle-CssClass="art-button"
FinishCompleteButtonStyle-CssClass="art-button"
FinishPreviousButtonStyle-CssClass="art-button"
NavigationButtonStyle-CssClass="art-button"
StartNextButtonStyle-CssClass="art-button"
StepNextButtonStyle-CssClass="art-button"
/>
It won't fix it right now but it will work after the next release of mojoPortal. I just came up with a solution that seems to work but it did also require a small code change in mojoPortal which is why ot won't work for you now, but if you put it in your theme.skin file it will start working after the next upgrade of mojoPortal.
In the meantime it could be done using a javascript approach as Isaac mentioned though you would probably need to view the source ofe the page to get the id of the button to use in the jquery selector.