These examples seem to be using Source Sans Pro, a new open-source font from Adobe that WRI has introduced for its new web pages. Although it's a good font for advertising copy, I'm not sure it's as successful as other fonts would be for any type of source code - and Mathematica code is as demanding as anyother language, with its rich supply of punctuation and characters from the less explored regions of the Unicode table.

Interestingly, Adobe also supply Source Code Pro as a monospaced companion to Source Sans Pro, specifically for coding purposes. WRI don't seem to have adopted it though. It seems quite good, at first glance.

Here's a picture of showing what code looks like in today's Mathematica, tomorrow's Mathematica documentation, and if you change your source code to, say, Source Code Pro (three weights shown, Regular, Medium, and Bold):

The Source Sans Pro used in the new documentation pages has the one advantage of taking up less space. But it doesn't seem suitable for code, to my eyes at least.

I don't think you'll be able to change the font used by the documentation - although wouldn't that be a useful option?