@mixin border-radius($radius) {
  -webkit-border-radius: $radius;
     -moz-border-radius: $radius;
      -ms-border-radius: $radius;
       -o-border-radius: $radius;
          border-radius: $radius;
}
button{
    background-color: $theme-color;
    border: 0px solid;
    padding: 0.7rem 3rem;
    @include border-radius(2rem);

}

http://sass-lang.com/documentation/file.SASS_REFERENCE.html#including_a_mixin