Differing text margins for different pages?

Hello all! ??
I am new to WordPress, and web development overall. However, I do understand basic PHP/CSS/HTML syntax being a programmer of many other languages.

I just got myself a site and theme, and have been working with configuring it to my liking. I currently have it configured to only display the sidebar on the main page, and not on any other page. However, I would like for the text to wrap much wider on the pages without the sidebar than the main page, as there is much more room.

Here’s my main page:
https://aidancbrady.com

Notice how the text stops immediately before it reaches the sidebar.

Content page:
https://aidancbrady.com/contact/

There is no sidebar there, so I’d like for the text to extend a bit further to the right.

I looked into the stylesheet, and I noticed that if I modified the margins in the “.content” section of the file I could get it to wrap further. However, this also made the text be wider on the main page, which it shouldn’t.

Here’s a quick screenshot of the code I was editing:
https://puu.sh/66YGW.png

I changed the width settings to 100%, which was perfect, but it also extended the text on the main page like I said. I’m sure there’s an easy way to do this, but I have absolutely no idea what to do! Any help?