Article page margin

Hello to all
I need to change the margin on article pages, current is 100px.
- Where and how to add/change the css code?
- Whats the code to add?
Please see attached printscreens.
Thank you!
this is the webpage: https://bombeirosbarquinha.pt/index.php/pt/inicio/em-destaque/328-bombeiro-e-levado-a-sua-ultima-morada1

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