How do I expand the width of the content area?

Please go to "Appearance > Customize > Additional CSS" and apply all the custom CSS below:

.post-title, .section-title {
    float: none;
    width: 100%;
}
.post-excerpt, .post-content, .section-content {
    float: none;
    width: 100%;
}
.fl-row-content-wrap {
    padding-left: 0;
    padding-right: 0;
}
.post-content-container.no-image {
    margin-top: 0;
}
.side-fixed #side-container {
    margin-top: 0;
}
.post-content a {
    
}
.post-content a > img {
    float: left;
    margin-bottom: 25px;
}
h1 a, h2 a, h3 a, h4 a, h5 a, h6 a {
    border: 0;
}
.post-excerpt, 
.post-content, 
.section-content,
.post-meta-wrapper, 
.author-social {
    float: none;
    margin: 0;
    width: 100%;
}
.post-excerpt, 
.post-content {
    margin-bottom: 25px;
}
.single-post  .post-content {
    margin-bottom: 50px;
}
.post-meta-wrapper {
    overflow: hidden;
}
.single-post .post-meta {
    margin-bottom: 5%;
    overflow: hidden;
}
.single-post .post-meta:last-child {
    margin-bottom: 0;
}
.post-meta li {
    float: left;
}
.post-meta li::after {
    content: '//';
    padding: 0 8px 0;
}
.post-meta li:last-child::after {
    content: '';
    padding-right: 0;
}
.meta-categories li::after,
.meta-tags li::after {
    content: ',';
    padding: 0;
    margin-right: 6px;
}
.meta-categories .meta-categories-title::after,
.meta-tags .meta-tags-title::after {
    content: '';
    margin-right: 0;
}
.meta-categories .meta-categories-title,
.meta-tags .meta-tags-title {
    float: none;
}
.post-content-container, 
.content-section-wrapper {
    
}
.top-section {
    margin-bottom: 6.5%;
}
.section-content {
    margin-bottom: 3%;
}
.comment-author-avatar {
    margin-right: 25px;
    width: 100px;
}
.blog-list .post-image {
    margin-bottom: 25px;
}
@media only screen and (max-width: 1350px) {
    #content-container {
        width: 70%;
    }
    
}

The code will set the content area to 100%, reduce the spacing at the top, and adjust some other elements to best work with this custom width.