Default theme should fallback to native font stack

[gma]

Target the system fonts of individual OS to remove the reliance on google fonts.

The motivation is captured neatly in commit 641e1688 in the NWRUG web site repo, with:

See the Using UI System Fonts In Web Design: A Quick Practical Guide Smashing Magazine article [1] for details on how this particular set of fonts were put together.

[1] https://www.smashingmagazine.com/2015/11/using-system-ui-fonts-practical-guide/