On the left panel select Settings > Advanced Settings. Then select Font under the "Look and Feel" section. This will change the Title's font throughout your feedly app.
Note: in order to properly display feeds for Chinese, Korean, Japanese, Cyrillic and most other non latin languages, you should switch your default font to the "roboto".
Click on Preference at the bottom of the left panel. You can select the headers' font under the "Header font" section.