Hi, I doubt customizing font is a possibility and even less choosing where it applies.
But I guess you can try and replace the current font by the one of your choice.
I have no clue where it is stored, but substituting the current font file with yours should be possible as a hack.
On a sidenote, we indeed haven't done our best yet to provide all the information about our needs for UX.
The website is currently going to be refreshed, so we'd ask you to stay tuned for further updates 😃
As you all properly pointed out, one of first goals is to provide consistency. Or, at an higher level, coherence.
At the moment we're approaching a transition from Ubuntu Vivid to Xenial, and we will be tied to a different toolkit (QtQuick Controls 2, shortly QQC2).
Our effort is to port the Suru design to QQC2; in this regard, we're creating a new style for the components. Here's a very early video, the style is based on Vanilla Design.
With the new toolkit, which offers less control on how components behave, and all the features that convergence brings, including the ability to run apps that hasn't been designed for Ubuntu Touch, we can expect some obstacle in fully implement the current set of specifications.
Our hope is that it could turn in an opportunity to refresh our Suru specifications.
The full documentation for the current Suru design is not available or, in some parts, outdated (mid-2013, when Suru was still using those odd gradients).
The same documentation available on docs.ubuntu.com has never been completed during the last four years.
In the meantime, we have seen the raise of new UX trends, one of the controversial components (the bottom navigation) has been officially adopted by the Material guidelines, and Canonical decided to drop its Ayatana project (i.e. Unity 7) and adopt the GNOME 3 HIG.
What we need is help and guidance through these multiple inputs, in order to fix some of the current issues in usability, and eventually validate the design of new UI components. In a longer term, we might want to support an effort to revisit the current design guidelines.