I'm sorry about the bad title, I simply don't know how to better phrase my question.
So, the thing is: I have a long Japanese .txt file. Given my current level of Japanese, I need to use a Chromium dictionary extension.
I can open the .txt file in the browser and use the extension, but with no formatting the text is not readable enough. I tried hosting it on a public folder in dropbox and using reformatting tools like Readability or iReader, but both won't work (they don't appear to work on .html pages from my public Dropbox folder either).
I also know about how Chromium lets you edit pages on the fly, and how one can apply custom CSS styles to pages, so I was wondering if it was possible to somehow make the text more readable¹ using such tools. The source text is in a .txt file, but I guess I could convert it to an html page using LibreOffice or something like that, if that's necessary.
So, in a nutshell: how can I display very long (a whole book, in fact) unformatted text in Chromium in a more readable manner?
 By readable, I mostly mean bigger fonts and wide margins, pretty much what stuff like Safari's Reader or the iReader extension would do. I obviously do not expect it to know what's a chapter title or anything like that.
P.S.: While I did use the term "CSS", I'm not completely sure that's what I need. If CSS has nothing to do with this, please forgive my ignorance. I know next to nothing about web... stuff.
P.P.S.: While I did talk about iReader to clarify what I need, using iReader itself is not an option since the dictionary extension (Rikaikun) does not like it very much.