In Site Pages library, I have created a new Site Page and added a Text Web Part. This web part shows only few predefined text styling like Normal Text, Heading 1,.... but there is no provision to change the font family.
Is there any other way or any suggested approaches?
Try doing what you would like to create in MS Word, then paste the text into your web part. When I do this it usually appears how I would like it to and keeps formatting from Word.
It’s by default we are not able to change the font in modern experience page in SharePoint Online.
Or you could apply custom theme to change the look of your SharePoint Online site.
font-styles defined in the site's style sheets (CSS). You need to add your custom CSS snippet to the page to customize the font styles from the defaults.