Updating the TODO on the chartResetButton

Ran into this issue today, not having a strongly defined theme for the reset zoom button. 
Didn't go any further than the first level as the states are branches of HTML elements which are not highcharts specific.

highcharts/highcharts.d.ts

@@ -191,7 +191,15 @@ interface HighchartsBoolOrShadow {
 interface HighchartsChartResetZoomButton {
     position: HighchartsPosition;
     relativeTo?: string;
-    theme?: any; //TO DO
+    theme?: HighchartsChartResetZoomButtonTheme; //TO DO
+}
+
+interface HighchartsChartResetZoomButtonTheme {
+    fill?:string; //css HEX colours. 
+    stroke?: string;//css HEX colours. 
+    r?: number; // Radius %
+    states?: any; // HTML element states eg: hover, with css attributes in object. 
+    display?:string; // css attr eg: 'none' 
 }
 
 interface HighchartsChartOptions {