I build custom themes all day long all year long. Recently I created a theme with the word Diamond in the name. Once it was installed in WordPress, WordPress wanted to offer an update for it. It was offering an update to some random theme named Diamond. I tried changing the name in the style.css file to have other words in it, but it seems just having the name diamond triggered it. Then I changed the version number to 100. Since that’s way above any likely version number in the WP database, WordPress stopped offering an update! Alternatively, you could just remove the version number. That works as well.