How do Tumblr theme developers know if you edit their themes/remove credit?

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or do they not know and just put that warning to scare ppl?

[Stone]

If you went to all the trouble to design a theme from scratch, and put it out there for people to use for free, you'd probably pay attention to how many people were using it... I know that I would...

It isn't that much trouble to find your theme(s)... And to check whether your credit is intact...

It's just bad manners to remove the credit... don't want someone's name in the source code?
Design your own custom theme. There are videos on youtube...

[Windowphobe]

It's possible that they've coded the theme to call home at some place other than the obvious one that you edited out. This is also done in WordPress.