If switching themes causes problems, the theme you’re switching from (and then back to) likely has bugs (i.e. report this issue to your theme author).
Switching to a default theme like Twenty Seventeen is to see if the .container img… code is still in the page source. If you don’t know how to check for this, please ping me once your site is running Twenty Seventeen, and I can look.
Please also remove that extra CSS you have added (note that if that’s exactly what you added, it has no selector so it wouldn’t apply to anything).
Of course, we always recommend restorable database and file backups as well as performing modifications first on a staging site before modifying anything on your live/production site.