You can perform a basic modification to the appearance of books to make your reading experience more comfortable for you. iBooks allow you to adjust the font size and fonts. You can choose from five included fonts. But, as of now, you cannot install custom fonts. Included fonts are:
- Seravek and
- Times New Roman.
You can also select from multiple color schemes: Normal, Sepia and Night. You can also tap General > Accessibility and toggle Invert Colors on/off; this will inverse the colors, making everything white on a black background (and vice versa when changed back).