1) You can use cascading selectors to find the inner thing; such as (or be even more specific if it doesn't match
.mendixButton span.inner_style { color : red }
2) you should be able to change the style on any thing from javascript, so problably your JS is incorrect there. Just use dojo.style (I think you can use dojo.query(xx).style({ color : 'red' }) as well I think