If the font has changed in your Yahoo Mail. Then as far as I understand, after inspecting the Yahoo Mail web page, Yahoo uses the following fonts in the following order depends on what font is on your system

YahooSans
Helvetica Neue
Segoe UI
Helvetica
Arial
Lucida Grande
sans-serif

If you installed any new fonts recently on your computer delete the ones that match the names above. Be careful what font you delete. ONLY delete the ones you installed. Don’t delete Segoe UI and Arial because it came with Windows. If you accidently replaced an existing font then try to copy the original font from another computer.