The definition of theft is found at Section 1 of the Theft Act 1968:
A person is guilty of theft if he dishonestly appropriates property
belonging to another with the intention of permanently depriving the
other of it; and “thief” and “steal” shall be construed accordingly.
That definition is broken down into 5 components which are detailed further in the Act here. Section 2 provides that appropriation is not dishonest if the person appropriating the property believes he has a legal right to deprive the other of it or that he would have the other's consent.
In contract law there is the concept of a unilateral contract. See e.g. Carlill v Carbolic Smoke Ball Company  EWCA Civ 1. This involves making an offer to "the world" (or some narrower class of offerees) which is capable of being accepted (thus forming a binding agreement) by fulfilling the terms of the offer. In Carbolic Smoke Ball, the offer was to pay £100 to anyone who caught a flu after using their product. In your scenario, the offer is to give an item to the first person who takes it.
However, in order for an offer of a unilateral contract to be valid, there has to be intention to be bound on the part of the person making the offer. Intention is determined using the objective test (what a reasonable outside observer would think was in the mind of the person making the offer). It's quite clear that intention is lacking in the boot sticker scenario. Thus, if you take the case, you have no reasonable basis to think there was consent or a legal right to deprive, and the act would constitute theft (provided all the other elements of theft are met).
If someone leaves an item of furniture on the street with a "take me" sign, it's clearer that there is intention to be bound and/or consent. With the bowl of sweets, there is unlikely to be intention. That's because of the (rebuttable) presumption that in personal contexts there is no intention and in business contexts there is intention. There would still be no theft however because there is consent for the sweets to be taken.