If what you are wanting to effect is the appearance of no-style paragraphs in the Editor pane(s), then you do this by setting the paragraph/character format preferences in Scriv prefs.
If what you are wanting to effect is the look of no-style paragraphs in compiled outpit, then you do thus by editing your chosen compile format for its the paragraph/character format defaults.
Far as I know, there is no global style set per se.
If you are trying to globally effect the editor, see (1) above – though this only sets the default paragraph style.
If you need more flexibility, make a project with the styles you want and save it as a template. Result is static in the sense that new projects just inherit whatever styles were in the template at the time of creation. Though, since it is easy to import styles from other projects, you can master project you could import/re-import styles as needed into your other projects.
Note, however, that the “default paragraph style” default will be removed in 3.0.3. This was added after a request during beta testing, but on reflection it was a bad idea. Scrivener’s styes system is built around the idea that styles are only used for text that is different from the bulk. Body text should have no style associated with it, as this allows much more flexibility during Compile (thus the preference has been removed to discourage users from applying a style to everything). You can set the default formatting for text that doesn’t use a style via the Preferences, under Editing > Formatting, as gr says. If you need your body text to have a style applied in the final document, this can be done via Compile - you can create a Section Layout that applies a “Body” or “Normal” style to text that does not already have a style associated with it.
KB, I think the users asking for this (like me) are mainly interested in changing the editor formatting to have a different feeling while writing (for example, setting a dark background with a white text, or switching between monospace or other fonts). So maybe the solution could be to make possible to overwrite the main editor styles and fonts, like happens for “compose mode” (but with font choosing too).
gr, thanks, I didn’t know I can import styles from other documents!
There reason there is no option to override the text colour in the main editor mode is that, because Scrivener is rich text, it could be confusing to have the real formatting of the document hidden at all times. However, with Apple’s new Dark Mode coming in 10.14, there will be a way of getting light text against a dark background.
It is not possible to override fonts in the same way as it is colour, though. You can change the font for the text, of course, and you can change the default formatting for documents (in the Preferences, as mentioned), but there is no way of saying, “Use this font for display while really having a different font underneath”. (This is just a limitation of Apple’s text system.) There should be no need for this, though - you can choose which font you want to use to write in, and then have it changed during Compile so that your final format uses a different one.
It is also the case that one’s compile format can be used to change how each style presents. That means that, in principle, one could on a style-by-style basis craft a wholly different look for styles in the editor from their look in compiled output.
I can’t imagine doing this but for a dedicated purpose, but there you have it.