Is it permissible to steal money to buy a hijab if the mother refuses and swears to prevent its wearing, with the intention of returning the amount later?

You are not permitted to obey your mother in abandoning the prescribed Islamic dress. Furthermore, whoever is responsible for your financial support must provide it for you. If your mother refuses to provide it, it is permissible to take from her money, without her knowledge, what suffices for the obligatory and appropriate clothing, as Ibn Baz, may Allah have mercy on him, issued a fatwa (definition) allowing taking from a father's money, in a reasonable amount commensurate with the need, if he falls short in providing maintenance.