Adding custome css code to CTC

I have created a file called full-width.php which I can FTP to my twentyseventeen-child directory with not problem. But now I need to add this new code to the CSS file in my child directory

.page-template-full-width .content-area {
width: 100%;
margin: 0px;
border: 0px;
padding: 0px;
}

.page-template-full-width .site {
margin:0px;
}

but I am not modifying an entry as far as I know in twentyseventeen so how do you create a new entry? I have change colors in various places by searching for color and editing where needed but this is a new entry in my css.

If there is a video or instructions some place that would be great as well.