You can turn 'px' to 'em' or 'rem' units Instantly. The default base is 16px but you can define a custom base for each conversion

First method:

1 - Type cmd+shift+E on the line to be converted.

Second method:

1 - Add the base value to the end of line. Example:

 margin: 18px; /12

2 - Type cmd+shift+E and the result is:

 margin: 1.5em;  /* 18/12 */

*You can choose the type of units you want to convert in the package configuration