Add clearStyleCache method

src/core.js

@@ -52,6 +52,10 @@ c3_chart_internal_fn = c3.chart.internal.fn;
 var nodeCss;
 var styleCache = {};
 
+c3.clearStyleCache = function(){
+    styleCache = {};
+}
+
 // Seek a specific value in the stylesheets of a document
 function getStyleValue($$, selector, property) {
     function cache(value){
@@ -97,6 +101,7 @@ function getStyleValue($$, selector, property) {
     }
     for (var styleId = 0; styleId < document.styleSheets.length; styleId++) {
         sheet = document.styleSheets[styleId];
+	if(!sheet.cssRules) return undefined;
         for (var ruleId = 0; ruleId < sheet.cssRules.length; ruleId++) {
             rule = sheet.cssRules[ruleId];
             if(rule.type !== CSSRule.STYLE_RULE) continue;