[Resolved] FIrst post full width

Hi there!

I’m starting to use the WPShowPost Pro plugin and I’ve seen that the featured post option doesn’t make the first post full width as GeneratePres does in Blog options.
I like the way GeneratePress theme makes the first post full width and I’d like to get the same results with WPShowPost Pro.
I see the css is using Flexbox but I’m not sure if it uses any kind of fallback to support older browsers.
Anyway, what css should I use to get full width in first post?

Thanks!
Daniel