Colour gender in profile postbit?

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Depends on the value of the girl and the boy in your custom profile field.
If it is a simple girl and boy, you can create trow1/2girl/boy class, then add the {$post['fidx']} after the trow1/2 class in your postbit.
Or, if you use Template Conditional, you can use it to manipulate the class used in postbit.