I've tried updating some of the site sections from theme editor (/wp-admin/theme-editor.php) but once I hit Update page, I got this change propagated to all PHP pages under this domain. Meaning, I do not have wp admin anymore. If I try to open any of it pages, instead of dashboard, of list of posts or whatever, I'm getting static page with picture.
How is it possible that editing anything through wp-admin's theme editor could break admin itself?
I requested reverting from backup from my hosting provider, but could someone help me figure out what I did wrong so that I don't do it again?