Better error message for undefined variables

examples/dev_sandbox.css

@@ -1,49 +0,0 @@
-/* CSS file for dev_sandbox.py */
-
-App > View {
-  docks: side=left/1;
-  text: on #20639b;
-}
-
-Widget:hover {
-  outline: heavy;
-  text: bold !important;
-}
-
-#sidebar {
-  text: #09312e on #3caea3;
-  dock: side;
-  width: 30;
-  offset-x: -100%;
-  transition: offset 500ms in_out_cubic;
-  border-right: outer #09312e;
-}
-
-#sidebar.-active {
-  offset-x: 0;
-}
-
-#header {
-  text: white on #173f5f;
-  height: 3;
-  border: hkey;
-}
-
-#header.-visible {
-  visibility: hidden;
-}
-
-#content {
-  text: white on #20639b;
-  border-bottom: hkey #0f2b41;
-  offset-y: -3;
-}
-
-#content.-content-visible {
-  visibility: hidden;
-}
-
-#footer {
-  text: #3a3009 on #f6d55c;
-  height: 3;
-}

examples/dev_sandbox.py

@@ -29,4 +29,4 @@ class BasicApp(App):
         )
 
 
-BasicApp.run(css_file="test_app.css", watch_css=True, log="textual.log")
+BasicApp.run(css_file="dev_sandbox.scss", watch_css=True, log="textual.log")

examples/dev_sandbox.scss

@@ -0,0 +1,58 @@
+/* CSS file for dev_sandbox.py */
+
+$text: #f0f0f0;
+$primary: #021720;
+$secondary:#95d52a;
+$background: #262626;
+
+$primary-style: $text on $background;
+$animation-speed: 500ms;
+$animation: offset $animation-speed in_out_cubic;
+
+App > View {
+  docks: side=left/1;
+  text: on $foo;
+}
+
+Widget:hover {
+  outline: heavy;
+  text: bold !important;
+}
+
+#sidebar {
+  text: $primary-style;
+  dock: side;
+  width: 30;
+  offset-x: -100%;
+  transition: $animation;
+  border-right: outer $secondary;
+}
+
+#sidebar.-active {
+  offset-x: 0;
+}
+
+#header {
+  text: $text on $primary;
+  height: 3;
+  border-bottom: hkey $secondary;
+}
+
+#header.-visible {
+  visibility: hidden;
+}
+
+#content {
+  text: $text on $background;
+  offset-y: -3;
+}
+
+#content.-content-visible {
+  visibility: hidden;
+}
+
+#footer {
+  text: $text on $primary;
+  height: 3;
+  border-top: hkey $secondary;
+}