Content Width Customization

Does Content Width customization require the “Custom Design” feature to be turned on? I have the Chateau theme and have been trying to figure out how to expand the Blog postings and content to something more than a narrow view (I am losing so much space based on this scrunched up view). Near as I can tell I have to have “Custom Design” before I can modify the CSS. I tried a Preview using the following from another Forum posting, but it didn’t seem to work and not sure if I must PURCHASE Custom Design before I can expand my layout?

Trying to Accomplish Against the Chateau WordPress Theme. Not having much luck 🙁

.singular #content {
margin-left:0;
margin-right:0;
}
.singular .entry-header, .singular .entry-content, .singular footer.entry-meta, .singular #comments-title {
width: 94%;
}