In b3, setting the General Text Attributes has no effect on the text in my Scrivener windows.
To support using MMD, I tried setting to Courier 15pt, with 2pt line spacing, and no indentation.
I quit Scrivener and restarted. No effect.
I copied my document to the clipboard, destroyed it on disk, restarted Scrivener, and pasted in the text. This time the font and spacing were present, but all paragraphs are still indented.