Can't target page margins

https://iamsinc.com/blog/12-bonus-9-mga-commission-learn-more-about-this-equitrust-product/

I want to remove the margin so that the word “test” (and anything I put on the page) goes all the way to the edge, next to the background.

I have tried targeting it numerous ways including
‘.page-id-1089 .singular #content {
margin:0;
}’

which I feel should be correct. I actually have that currently on my Child stylesheet, but it’s not having any effect. I want to adjust the margin for just this page. Thank you!