Css child theme customization

Hallo team,

I have a question regarding customizing my pages. I followed instructions on some sites (using Chrome inspector), copied it in my style file in WellNess Pro theme (Genesis Child theme), but it did not work. I also tried to generate codes using https://css3generator.com/. I am completely hopeless and confused, as I do not know to which place should I place them, whether to overwrite them.
I would like to ask you, if you could possibly advise me on where to paste the code (I was suggested to add it at the very end of the style file, but no change occurred).
This is what I′d like to do on my https://moreyouthfulskin.com/:
1) customize the content boxes (make them smaller and put the text centered)
2) put the flags in the header under the menu (centered)

Thank you very much for your help in advance.
Kind regards,

Milada