I have found a solution, but you have to edit every epub you have with Sigil.
The problem is that the order I put previously in the css, and was used to get the letter type:
.ePUB {font-family: "Helvetica Neue", Helvetica, Verdana, Arial, Trebuchet MS, Trebuchet, sans-serif; font-weight:normal;}
Is no more useful, because the new firmware ignores it. But if you get inside the line that stablish the characteristics of the general text (it is not very difficult to see it), the only thing you have to do is put the type of letter you want inside that line.
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