How can I revert original settings, e.g. for fonts, key bindings, etc.?
Sylpheed keeps its configuration files in $HOME/.sylpheed-2.0/sylpheedrc (or %APPDATA%\Sylpheed\sylpheedrc under Windows) and creates default entries, if none are found. So the easiest way is to quit Sylpheed, temporarily renaming your $HOME/.sylpheed-2.0 (or %APPDATA%\Sylpheed\sylpheedrc) e.g. to *.bak and restarting Sylpheed in order to get default entries.