You can change the display fonts in web sites by injecting some CSS. AddCSS addon do this work.
You can see a screenshot for a working example for this addon http://i.imgur.com/2gvwPUu.png
I changed the default paragraph font of Opera Forums to 'Amor Sans Pro'.
Here is the link for the AddCSS addon: https://addons.opera.com/tr/extensions/details/addcss/?display=en