I have no experience with changing fonts and tampering with css, sorry.
However, I'm curious, what do you mean the reader is crashing? Mine has never crashed as long as I've had it. It does however take an awful lot of time reformatting some ebooks when you change the font size. Maybe it's just that you don't wait long enough for it to display the changed font size? It looks like it's frozen when it's doing it.
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