Extend Post Width

Try not to bump threads, not only because it’s against the pirate code but because you posted on an empty thread you decreased its visibility to the community. Empty threads are monitored on a special “no-replies list” and some volunteers prioritise answering those threads over other non-empty threads.

Back to the issue, does the Twenty Fourteen theme come with a “Custom CSS” part of the dashboard?

Cool, assuming that means Twenty Fourteen doesn’t have a “Custom CSS” part of the dashboard install that Custom CSS Manager plugin.

Then use its section of the dashboard to hold your CSS modifications:

.full-width .site-content .entry-header,
.full-width .site-content .entry-content {
 margin-left: 0;
 max-width: 100%;
}