How change article’s (blog) CSS ?

Hello,

Thks to reading me.

1) I am looking to change the layout of my articles, without “killing” de SEO optimization.

Actually, my articles look like that : https://www.lamethodestreet.fr/musculation-pour-pro-gamer-et-streameur-esport/

With an basic H2 in color, and H3 with a gray background color.
It’s simple, but i find that it doesn’t highlight my text.

Should i use a plugin, how can i do that ? Or can i basically change the CSS anywhere ?

2) I want add more articles of the “Blog” page : https://www.lamethodestreet.fr/category/articles/

Actually, I have 12 items that are displayed, i want the double, where is the option to change that ?

Thanks in advance for the help.

Best regards,
L.

The page I need help with: [log in to see the link]