Bug 1311789: Fix the missing "Themes" legend in devtools options panel. r=jryans

MozReview-Commit-ID: 6ZpXntxvk3w

--HG--
extra : rebase_source : d129842a1dc81cce318e8e4e9e78ad5455bca8e4

devtools/client/framework/toolbox-options.js

@@ -256,7 +256,10 @@ OptionsPanel.prototype = {
 
   setupThemeList: function () {
     let themeBox = this.panelDoc.getElementById("devtools-theme-box");
-    themeBox.innerHTML = "";
+    let themeLabels = themeBox.querySelectorAll("label");
+    for (let label of themeLabels) {
+      label.remove();
+    }
 
     let createThemeOption = theme => {
       let inputLabel = this.panelDoc.createElement("label");