REM units, settings and variables

It would be great to be able to set the REM value per frame/section too.
If the default REM is set to 16 px it would be great to be able to side-step that for a section that is mobile frames for example.

One option could be to be able to change the REM value at the code settings, and change the value.
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It would also be a great feature if the variables would be possible to add math functions. For example that a [variableA] = 3*[variableB].
I know there are plug-ins for that but if that would be part of the basic FIGMA feature regarding variables.