If the text is still legible in the note list, that suggests that if you remove the custom styles, your existing notes (entered vertically) should remain legible too. Unfortunately it is not yet possible to switch individual notes, but given your valuable input I think we can go ahead and implement this soon.
We had some concerns regarding scrolling: for Japanese vertical input, would you expect the lines to break if they reach the bottom and, respectively, the note pad to scroll horizontally when you reach the left border?
Here is a silly hack you could use to switch between the writing modes: if you rename
style.css to either
style-dark.css the styles would only be loaded for the respective theme. This would allow you to switch the note pad's orientation by changing themes. As I said, that's just a silly hack, but the screenshot above is really encouraging and I hope we'll add a way to switch modes for individual notes within the next few weeks.