This example shows how you can convert all two column styles to 100% width single column styles.

twentyseventeen-full-width-columns.css

.site-content .wrap,
.has-sidebar:not(.error404) #primary,
.has-sidebar #secondary,
body:not(.has-sidebar):not(.page-one-column) .page-header,
body.has-sidebar.error404 #primary .page-header,
body.page-two-column:not(.archive) #primary .entry-header,
body.page-two-column.archive:not(.has-sidebar) #primary .page-header,
.blog:not(.has-sidebar) #primary article,
.archive:not(.page-one-column):not(.has-sidebar) #primary article,
.search:not(.has-sidebar) #primary article,
.error404:not(.has-sidebar) #primary .page-content,
.error404.has-sidebar #primary .page-content,
body.page-two-column:not(.archive) #primary .entry-content,
body.page-two-column #comments {
  max-width: 100%;
  width: 100%;
  float: none;
  padding-left: 0;
  padding-right: 0;
}