When I create the text, then change the font family, no change occurs. It stays the default and same. What to do, I want to use custom fonts?
Try specifying the needed font directly in “Font Family” field, e.g. “C:\Windows\Fonts\cour.ttf”.
Thanks @svarov But there is a question, does Synfig only supports .ttf font extension?
Yes, only ttf is supported. At least from what I know.

Synfig has not so much features for text. Not much useful when it comes to motion graphics. Developers should bring more support for text elements. I have to export text as png.
Otherwise thanks
Copy the font file you want to use from C:\Windows\Font and paste it in a new folder. Specify the address of the font in the font family