I have many visuals, titles, texts, legends and all are in different Font Family. How can I change them by Switch Theme Option? I mean modifying the json the I import into Power BI and that it set by default a Font Family for everything in my report. Also I want to set a color for each text in my report.
With Report Themes you can apply a color theme to your entire report. You can import your own JSON file to change the default settings. For more details, please see: Use Report Themes in Power BI Desktop