Default font in 2012?

Easest way without investing too much effort and not worrying about coding ethics (like clean code and so on) you could simply use the “Customizer”.

In your frontend, click on the “Customizer” in the upper left corner in the admin bar.
Then, scroll/go to the “Additional CSS” section and add

body {
    font-family: yourFontFamily!important;
}

to the customizer.
It will overwrite existing rules, unless each paragraph has explicitly set a custom font family. In that case this solution won’t work.

Otherwise, adding this to the customizer should do the trick.
It’s quick – and it’s dirty.