Add something along the lines of #ifndef

[foca]

Something along the lines of

#ifndef something
#define something value
#endif

…but less verbose. Maybe #noredef (thinking of noremap in Vim.)

The idea being that plugins or modules can define variables with default values, and you could override them before including the file, so:

// In some_plugin.css
#noredef $some-plugin-color #ddd
strong { color: $some-plugin-color; }

// In main.css
#define $some-plugin-color #ccc
#include "some_plugin.css"