Already asked? Applying new theme

I respected all the steps. I have no problems with the ftp, nor the process itself. I chmodded wp-content so that “If this file were writable you could edit it.” in the theme editor disappeared, but it didn’t. So no matter what theme I select, it just doesn’t work.
Can you please kindly show me where this had been already asked and resolved if it was, or help me through this.
Thank you.