rem
Represents the font-size of the root element (typically
<html>).
When used within the root element font-size, it represents its initial
value (a common browser default is 16px, but user-defined preferences
may modify this).
Source
In other words, change the font size of your html element, and the calculation base for rem will change.
Example:
<html style="font-size: 10px">
...
</html>