1. You can only use TrueType fonts (.ttf) on the reader. Appendix A in the guide mentions that you need to convert the OpenType font (.otf) to TrueType (using FontCreator for example.) The you can use SonyFont to create the final font.
2. Fonts defined in the custom css file are used only for epub files, while the default fonts are used everywhere. The sans font in the reader's interface (Home Screen for example) and the serif font for the book content (txt, lrf, epub).
The font from custom css overrides the default font, but you can still have a font embedded in the book that overrides the previous two.
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