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Look out for the API parameter, profile.aspect.theme in the API endpoint "Create a profile". Setting this parameter to one of the available values from the enumeration will ensure all web forms generated will go with the selected styling(theme).
Things to remember
Always test how what the theme will work with the rest of the profile parameters and (if set) the translationWeb Form will finally look like, with all the customizations. This includes profiles and translations. There is a chance that the customer will need to optimize the various values to have the best possible results from all the customization.
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