Changing the text style in the HTML is possible thanks to Cascading Style Sheets (CSS), the stylesheet language used to describe the presentation of a document in HTML. There are several ways to ...

How to Change Body Text Font on Tumblr. ... Copy the code from the font website and then paste the HTML code for the style sheet inside of the tag for your blog.

Fortunately, Tumblr enables you to change the font size for your blog's text by giving you direct access to the raw CSS or Cascading Style Sheet code of your theme. Log in to your Tumblr blog.