The refusal of B to give something borrowed from C back to C is not automatically theft. In some cases it might have to be resolved using the Civil Courts and you may need to see a solicitor for advice.
B borrows C's power drill. B keeps promising to return it, he does not intend to keep it and it is just left in the garage, but he doesn't get round to giving it back. If this is pure forgetfulness, which goes on for a fairly long period (despite reminders) it would not be theft. However, eventually there comes a point where C has been deprived of his property for so long that B has no excuse for not returning it (perhaps a year or more) and the Criminal Courts would consider he had stolen the drill.
If you have a problem with someone who has borrowed your property, keep a record of all the occasions you have asked for it back and then ask a solicitor to send a formal letter asking for the return of the property. If the property is still not returned, there may sufficient to justify making a complaint of theft to the police.
Every case will be different and it may be, for example, that the borrower claims the property was a gift. In circumstances like that, the police may not be willing to take action and you must to go to the Civil Courts to get your property back. In the case of a borrowed power drill this may not be economically worthwhile. Below is a brief summary of the offence of theft, it is not intended to be a comprehensive explanation.
Theft occurs when someone dishonestly appropriates (takes possession of or make use of exclusively for oneself/someone else without permission) some property that does not belong to him or her and treats it as his or her own and has no intention of returning the property to its rightful owner.
B is walking along the street chatting on her mobile phone when C runs past and takes the phone from her hand and runs off with it.
B takes an item from the shelf of a shop and leaves the shop without paying for it (shoplifting).