As a side-off note, Infopath 2003 used JS for code-behind though probably it wouldn't help you in any way since, I guess, you one should understand that Infopath forms are not HTML or web pages, they are XML data to be used by external tools, like for example Sharepoint Server IPFS - Infopath Forms Service, to generate web pages on the basis of XSN Infopath form template, while the data, in case of sharepoint library Infopath forms, is stored in Infopath XML data forms.
If to guess a little further what you rellay ask and go outside of OOTB then you might find helpful:
As it was answered before, it is possible to use C# or VB.NET in Infopath 2010 or 2013, unless it is Sharepoint List Form type where code-behind is not possible at all, and one cannot add OOTB CSS rules neither to Infopath XML data forms not to XSN Infopath form templates.
If you want to go non-OOTB and unintended ways, please check:
- Greg Collins. Add a Custom Style Sheet to a View
And if to go in intended and unproblematic way, then check: