allow negative numbers multiplication

src/value/ops.rs

@@ -214,6 +214,7 @@ impl Mul for Value {
                 }
             },
             Self::BinaryOp(..) | Self::Paren(..) => self.eval()?,
+            Self::UnaryOp(..) => (self.eval()? * other)?,
             _ => return Err(format!("Undefined operation \"{} * {}\".", self, other).into()),
         })
     }

tests/values.rs

@@ -463,3 +463,8 @@ test!(
     "a {\n  color: (a b) - (1 2);\n}\n",
     "a {\n  color: a b-1 2;\n}\n"
 );
+test!(
+    negative_number_times_number,
+    "a {\n  color: -1 * 2;\n}\n",
+    "a {\n  color: -2;\n}\n"
+);