Am I obligated to return 1000 dirhams to my brother, which I took from his store without his knowledge, given that he did not pay me for an entire year that I spent working with him, and is this action considered theft in Islamic Sharia?

You must repent sincerely and immediately stop abandoning prayer, for it is a grave sin. The Prophet, peace and blessings be upon him, described its abandoner as an disbeliever. Be diligent in your prayers to solve your problems and attain happiness in this world and and the Hereafter, and so that they may restrain you from indecency and evil. Be frank with your brother about what you took, and kindly and lovingly ask for your right, and preserve the bonds of brotherhood and kinship. And note that the last statement you mentioned, attributing it to the Prophet, peace and blessings be upon him, is not authentically attributed to him; rather, it is from the words of Ibn Mas'ud and Ibn Abbas, may Allah be pleased with them both.