Editing the theme files is no longer needed, as version 2.5 includes an in app theme editor. The themes are still stored as JSON, so I’m keeping this post for reference.
Theme File Format
Fiery Feed theme files use a JSON like syntax, in that it is JSON, but also allows comments. The file needs to be UTF8 encoded.
Theme files are JSON files nending in .fftheme. All other files will be ignored.
The colors are written down as hex colors and support an optional alpha channel, either in the form of
RRGGBBAA, without the usual leading ‘#’. Empty strings will be treated as completely transparent. Other values are simple floats or booleans.