Add initial dark mode style

src/main/resources/css/dark.css

@@ -0,0 +1,20 @@
+/*
+JavaFX CSS Reference Guide
+https://openjfx.io/javadoc/23/javafx.graphics/javafx/scene/doc-files/cssref.html
+*/
+
+.root {
+    willow-background: #2D2D30;
+    willow-foreground: #ffffff;
+
+    -fx-background-color: willow-background;
+    background-color: willow-background;
+}
+
+.label {
+    -fx-text-fill: willow-foreground;
+}
+
+.radio-button {
+    -fx-text-fill: willow-foreground;
+}