General reports on the 'net indicate that plugin_host can crash for various reasons, like a recently-updated plugin with a major error in it. In those cases, the general advice seems to be to disable suspect addons one by one to identify the culprit.
For users with a custom theme and Package Control, though, the culprit is often simple: there's a package control bug that can sometimes just spontaneously disable plugins. When that one of those plugins is your theme, the interface will break and the plugin_host will report that it has crashed.
For now, Sublime Text is operating well enough though. Go to Preferences then Settings — User, find the "ignored_packages" entry, and see if your theme is in there:
In this case, since Seti UI is in there, we'll select it and delete it. Don't forget to also delete the comma on the line beforehand: the last line in the "ignored_packages" list must not end with a comma, but the rest of the lines should.
After deleting that line, save your file, and suddenly Sublime Text should be looking normal again:
(This answer was based on a solution from qgates on the sublime text forums that worked for me, and I'm reproducing the solution here for posterity + ease of finding it.)