I suggest adding note styles, selectable like edit/view mode is currently selected.
Currently, custom CSS files are limited to create 2 global styles - dark theme and light theme, selectable from settings.
I suggest adding possibility to define additional styles in custom CSS, only redefining note rendering (and applying over themes), with those styles easily selectable without changing settings and theme (from the note title bar, 4example).
It would make it possible to create different view modes for
- editing (as with WYSIWYG different view/edit modes would be gone)
- indented outline-like view (see Css-powered outline-like mode suggestion)
- anything you can imagine.
It seems to be in line with the current HTML and CSS model and should not cause compatibility issues for current CSS files.