From the docs: http://sass-lang.com/docs/yardoc/file.SASS_REFERENCE.html#variable_defaults_
You can assign to variables if they aren’t already assigned by adding
the !default flag to the end of the value. This means that if the
variable has already been assigned to, it won’t be re-assigned, but if
it doesn’t have a value yet, it will be given one.
For example:
$content: "First content";
$content: "Second content?" !default;
$new_content: "First time reference" !default;
#main {
content: $content;
new-content: $new_content;
}
is compiled to:
#main {
content: "First content";
new-content: "First time reference"; }
Variables with null values are treated as unassigned by !default:
$content: null;
$content: "Non-null content" !default;
#main {
content: $content;
}
is compiled to:
#main {
content: "Non-null content"; }