When I add code to my child theme, it doesn't change the site

Great! Glad I could help.
I dont have time at the moment to look at the spacing but I will get back to you later.

Thank you ??

Hmmm,

I am not seeing the changes of the width at https://www.greenh.net/

But you are?

Yes ??
Do you know how I could lose the background, i.e, have a full width page?
Thank you again

Set the width to 100%
I do see the wider site now

Between text and title you can add

.entry-title {
padding-bottom: 0;
}

But there are quite a few margins and paddings you can adjust for the things you want to change.

I would suggest you check each part with Firebug and you will be able to see.

PS it is generally not good to have a 100% width because on large monitors your site will look too wide.

I think there’s a problem with my server’s cache tool, because I turned off every option and it still doesn’t update for hours.
It is very difficult to change anything without imediate feedback.
I’m really sorry for all the trouble you’ve been having.
I decided to take your advice and I won’t make it full width.
I guess I’m getting there!
Thank you so much for all your help :)!

Glad I could help.
Further success!