Possible to update global styles programatically?

Is it possible to update the values in global styles programatically?

I have some styles like:

<Style TargetType="Label">
    <Setter Property="FontSize" Value="25" />
</Style>

And I wish to be able to change the value of the FontSize property in code. Doing so would ideally update all labels to use the new FontSize.

Does anyone know if this is possible and if so what would be the best approach?