In Jupiter X you can change the default css font properties for main body. You can modify Font-Family, Font-Size, Text-Color, Font-Weight and other body's css style properties.
In order to change body's css style properties:
1. From the WordPress left dashboard, go to Appearance > Customize or click on Customizer from Wordpress Admin bar.
2. From the Customizer left dashboard, go to Font & Typography menu.
3. In Typography Tab, select Customize for Body.
4. Change body's css style properties.
5. Click on Close and Publish the changes.