SharePoint 2013 theme broken accidently

In SharePoint 2013 if I try to change the look, and select "Tokyo" from the Site Layouts dropdown, the elements in the preview are all broken.

Menu items are appearing on the page which should only appear on hover, ribbon is broken, names like "BrowseTab 1 of 3, PageTab 2 of 3 SiteTab 3 of 3" are appearing.

I have not deleted any css or js or aspx file, only created a minimal master page and added some snippet to it.

I do see a 404 in FireBug: https://ourdomain.sharepoint.com/_catalogs/theme/Themed/E7642FC7/tokyo-B210DD14.themedcss?ctag=32

I would like to restore Tokyo site layout back to its original form.