Was there ever a Mobile First Template for Coffeegrinder for RSD ?
I don't recall there ever having been mobile-first for coffeegrinder.
Regarding the pixels to REMs auto-conversion, the matter is easily seen with border widths. The Design panel gives no option to set border widths other than in pixels. When one goes to the Inspector panel, the CSS(READ ONLY) section reveals that the CSS shows it in REMs.
I discovered the above long ago in connection with making shapes with containers. Because printers do not print background colours, I used wide coloured border settings to fill shapes. That seemed perfect until I displayed in one little known browser (Midori), where the REM conversion severely distorted some shapes.