Hi,
You can use CSS to do this sort of things. My best advice is to open up the Chrome Dev Tools, check what HTML gets produced by Orbeon Forms, and right there in the browser change the CSS to make it look the way you want. Once you are happy with it, save it to a file, which you add to your forms. For forms you create with Form Builder, see:
http://wiki.orbeon.com/forms/doc/developer-guide/configuration-properties/configuration-properties-form-runner#TOC-Adding-your-own-CSS-files-Orbeon-Forms-4-Alex
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