"theft n. the generic term for all crimes in which a person intentionally and fraudulently takes personal property of another without permission or consent and with the intent to convert it to the taker's use (including potential sale)."
I gather this is USA law? I'll work with it anyway.
Fraudulenty: by not paying, that's fraudulent.
Personal property of another: it is legally the property of the shop.
Without permission or consent: you could make a really shaky argument for implied consent here, but I don't think it would stand up.
To convert it to the taker's use: the intended use of the rightful owner (that's Walmart) was not to use it in the betta cups. Whether you use it in the shop or not, it's still for the taker's use, not the use of the shop. It doesn't matter that that's what the use of the shop should have been.
It's theft. I'm a law student. I do this sort of problem every day. If the law was a moral instrument, it would be on your side, but it isn't. Law and morality rarely have much to do with each other. :(