js,gg: more work on porting gg to JS backend (#12903)

vlib/gx/color.v

@@ -232,3 +232,7 @@ const (
 pub fn color_from_string(s string) Color {
 	return gx.string_colors[s]
 }
+
+pub fn (c Color) to_css_string() string {
+	return 'rgb($c.r,$c.g,$c.b)'
+}