Add option to overwrite and redeclaration warning option

src/commonMain/kotlin/nl/astraeus/css/style/CssFunctions.kt

@@ -0,0 +1,11 @@
+package nl.astraeus.css.style
+
+import nl.astraeus.css.properties.Color
+
+object CssFunctions {
+
+    fun darken(color: Color, percentage: Int): Color {
+        return color
+    }
+
+}