- Add Comic Sans MS as a test font option to verify font switching works
- Add !important to background-color and color in getCSS() to prevent
book CSS from overriding user's reading theme colors
- Expand font size range from 12-24px to 10-40px, bump default to 18px
- Expand line height range from 1.0-2.5 to 0.8-3.0
- Add restoreDefaults() method that resets reading theme, font, size,
line height, and justify/hyphenate to sensible defaults
- Add ↩️ restore defaults button in top bar underneath the settings gear