Declare closure-stylesheet mixins (3-typography.gss) and try to use them (in 5-main.gss)

3-typography.css

/* Typograph Vertical Rythms  via http://drewish.com/tools/vertical-rhythm 
Initial params:
 Georgia - base font-family,
 15px - base font,
 22.5 - line height ,
 40, 35, 30 20 15 13  - target fonts rythms
  --------------------*/

@def _FNT_SIZE    15;
@def _EM  _FNT_SIZE;

body {
  font-size: mult(1px,_FNT_SIZE); 
}

 p {
  font-size: 1em; /* 15px */
  line-height: 1.4667em; /* 22px */
  margin: 0 0 1.4667em 0;
}

@defmixin small() {
  font-size: 0.8667em; /* 13px */
  line-height: 1.6923em; /* 22px */
  margin-top: 1.6923em;
  margin-bottom: 0em;
}
.small {
  @mixin small();
}

@defmixin normal() {
  font-size: 1em; /* 15px */
  line-height: 1.4667em; /* 22px */
  margin-top: 1.4667em;
  margin-bottom: 0em;
}

@defmixin big() {
  font-size: 1.3333em; /* 20px */
  line-height: 1.1em; /* 22px */
  margin-top: 1.1em;
  margin-bottom: 0em;
}

.big {
  @mixin big();
}

@defmixin large() {
  font-size: 2em; /* 30px */
  line-height: 1.4667em; /* 44px */
  margin-top: 0.7333em;
  margin-bottom: 0em;
}

.large {
  @mixin large();
}

@defmixin huge() {
  font-size: 2.3333em; /* 35px */
  line-height: 1.2571em; /* 44px */
  margin-top: 0.6286em;
  margin-bottom: 0em;
}
.huge{
  @mixin huge();
}

@defmixin gigantic() {
   font-size: 2.6667em; /* 40px */
  line-height: 1.1em; /* 44px */
  margin-top: 0.55em;
  margin-bottom: 0em;
}
.gigantic {
  @mixin gigantic();
}

5-main.css

h1 {
    @mixin large();
}