The provided themes in 8.0.13+ declare Noto Sans as the default typeface. To use Roboto instead, you’ll need to treat it as any other custom font.
Your project needs to understand what the prop ‘Roboto’ means before it can find and use the relevant font files. This means adding webfont files (TTF, WOFF, etc) and mapping them per weight or variant of the font which you plan to use (italic, bold, medium, light, 400, 700, etc) in a theme. By doing this at the theme-level, it will be the only change you need to make for your project to use this font everywhere.
Read up on the custom theming process here
That theming README can be found in this folder:
You can grab the Roboto font files from Google here.
I hope this is helpful in getting you started. To verify, I just made a “light-roboto” theme locally, it was quite straightforward and a quick process (thanks to our amazingly talented front-end developers)!