Consider the following CSS snippets:

$x: 
* {
    background: red;
}

$x:
* {
    background: $x;
}

* {
    background: red;
}
$x:

All three should work with the following behaviour:

• an empty variable definition is allowed; and
• setting a style to an empty variable should unset that style.

For example, 2. should unset the background of all widgets.

As of now, 2. and 3. can't be parsed.
2. raises an error regarding an undefined variable and 3. raises an AttributeError inside substitute_references in parse.py.