2014 and 2015 themes are not very good / successful, I would say. Especially the 2014 one, which starts from the left hand side of the screen, and the other side is empty / blank. Somebody messed it up, for some reason, but people seem to use it, anyway. I centered 2014, one time, but it is still not good (proportions of post content to the rest of the design are not correct, I am not sure, again, they messed it up).
2015 is not bad, and it can be modified to anything you want, but it may be difficult, I am not sure. Some changes are easy, the other ones a little harder, but in general it is not a difficult thing to do. I worked on modifying 30+ themes, as child themes.
Edit:
Go to -
https://twentyfifteendemo.wordpress.com/
Right click on the left hand sidebar, and select "Inspect element".
Chrome Webmaster Tools opens, and you can start working on it with the magnifying glass, and the other tools there. It looks like "sidebar" div, is what you would be interested in. the @media screen may be getting in the way of this, a little bit, like I said, it may require some work. Anything can be done, in the end...
Also, like I said before -> google.com -> "twenty fifteen wordpress no sidebar"