i used to have a custom style for “normal” text, so that i could re-style it with new fonts and anything easily.
but i read in the manual that you suggest to use the “no style” for common text.
but then: how to apply a new default style (say change font and font-size) to all not styled texts?
i don’t want to go manually through all scriveners
Why would you want to apply a new font etc to all text in the editor?
You could change the default font in Preferences and then select all text and Convert to default formatting.
because style is important!
sometime i decide later the scripting font i want to use, or want to change it.
and when you have >200 documents, you want a consistent design everywhere
(it’s what i really miss from “simpler” editors like Highland or Ulysses)
I change how my normal writing font looks around twice a year. I just go through font phases, there’s nothing wrong with that in my opinion. Fortunately, the reason why we encourage leaving body text unstyled is that the tools for format conversion (in both the creative writing side and the compiler) are set up to assume that state.
Here are some tips from the knowledge base.
I too change up my writing font (“no-style”) periodically just to keep my eye sharp for errors. (Besides, I get bored. 
) Here’s how:
First, in Scrivener > Preferences, click on Editing in the top bar. Then click on the Formatting button. Set up your new “no-style” in the sample text area with the ruler. This sets the “no-style” for new documents.
Second, in the Binder, explicitly select all the documents you want to change to the new “no-style”. (Just selecting a container won’t work.) Then use Documents > Convert > Text to Default Formatting… . This brings up a dialog box in which you can preserve certain types of formatting. Click OK and you’re done!
See screenshots below. Hope this helps!
