Is it possible to use Selfish Mining to steal bitcoins? Is it possible to use the Selfish Mining algorithm to steal bitcoins?
Selfish Mining cannot be used to change transactions in any way, therefore, it is impossible to steal funds in such fashion. 

Also, Selfish Mining is somewhat the opposite of Double-Spending, as double-spending would require you to publish your own blocks as quickly as possible in order to overtake the network, while selfish mining is based on keeping blocks private as long as possible.

Since successful selfish mining makes another already found block obsolete, one might argue, that the Selfish Miner steals money from the miners that found the obsoleted block. However, in order to perform Selfish Mining, a new block has to be found in the first place, so the mining reward would have been the selfish miners anyway, if he had published his block right away.

Lastly, selfish mining forces the network to continue wasting their cycles on an already outdated block height. Yet, while their effort is wasted in a successful selfish mining attempt, this may better be described as obstruction than as theft.

Altogether, I can't think of a way that selfish mining would be used to perform anything that might be labeled Theft.