use system-ui font for font-sans
use system-ui font for font-sans

see https://www.smashingmagazine.com/2015/11/using-system-ui-fonts-practical-guide for the implementation details.

Signed-off-by: J Huang <042dc4512fa3d391c5170cf3aa61e6a638f84342@jhuang.me>

tommy351/hexo-theme-landscape,sandinmyjoints/hexo-theme-landscape,takeshik/hexo-theme-landscape,howiefh/hexo-theme-landscape-f,sandinmyjoints/hexo-theme-landscape,1PxUp/Naji,1PxUp/Naji,takeshik/hexo-theme-landscape,howiefh/hexo-theme-landscape-f,hexojs/hexo-theme-landscape