--- a/browser/components/customizableui/CustomizeMode.jsm
+++ b/browser/components/customizableui/CustomizeMode.jsm
@@ -1353,38 +1353,35 @@ CustomizeMode.prototype = {
           tbb.setAttribute("defaulttheme", "true");
         } else {
           tbb.setAttribute("image", aTheme.iconURL);
         }
         if (aTheme.description)
           tbb.setAttribute("tooltiptext", aTheme.description);
         tbb.setAttribute("tabindex", "0");
         tbb.classList.add("customization-lwtheme-menu-theme");
-        tbb.setAttribute("aria-checked", aTheme.isActive);
+        let isActive = activeThemeID == aTheme.id;
+        tbb.setAttribute("aria-checked", isActive);
         tbb.setAttribute("role", "menuitemradio");
-        if (aTheme.isActive) {
+        if (isActive) {
           tbb.setAttribute("active", "true");
         }
         tbb.addEventListener("focus", previewTheme);
         tbb.addEventListener("mouseover", previewTheme);
         tbb.addEventListener("blur", resetPreview);
         tbb.addEventListener("mouseout", resetPreview);
 
         return tbb;
       }
 
       let themes = [aDefaultTheme];
       let lwts = LightweightThemeManager.usedThemes;
       let currentLwt = LightweightThemeManager.currentTheme;
-      // The lwts besides the builtin themes don't have an isActive property:
-      for (let lwt of lwts) {
-        if (!lwt.hasOwnProperty("isActive")) {
-          lwt.isActive = !!currentLwt && (lwt.id == currentLwt.id);
-        }
-      }
+
+      let activeThemeID = currentLwt ? currentLwt.id : DEFAULT_THEME_ID;
 
       // Move the current theme (if any) and the light/dark themes to the start:
       let importantThemes = [LIGHT_THEME_ID, DARK_THEME_ID];
       if (currentLwt && !importantThemes.includes(currentLwt.id)) {
         importantThemes.push(currentLwt.id);
       }
       for (let importantTheme of importantThemes) {
         let themeIndex = lwts.findIndex(theme => theme.id == importantTheme);

--- a/browser/components/customizableui/test/browser_1007336_lwthemes_in_customize_mode.js
+++ b/browser/components/customizableui/test/browser_1007336_lwthemes_in_customize_mode.js
@@ -40,28 +40,50 @@ add_task(async function() {
      "There should only be three themes (default, light, dark) in the 'My Themes' section by default");
   is(header.nextSibling.theme.id, DEFAULT_THEME_ID,
      "The first theme should be the default theme");
   is(header.nextSibling.nextSibling.theme.id, LIGHT_THEME_ID,
      "The second theme should be the light theme");
   is(header.nextSibling.nextSibling.nextSibling.theme.id, DARK_THEME_ID,
      "The third theme should be the dark theme");
 
+  let themeChangedPromise = promiseObserverNotified("lightweight-theme-changed");
+  header.nextSibling.nextSibling.doCommand(); // Select light theme
+  info("Clicked on light theme");
+  await themeChangedPromise;
+
+  popupShownPromise = popupShown(popup);
+  EventUtils.synthesizeMouseAtCenter(themesButton, {});
+  info("Clicked on themes button a third time");
+  await popupShownPromise;
+
+  let activeThemes = popup.querySelectorAll("toolbarbutton.customization-lwtheme-menu-theme[active]");
+  is(activeThemes.length, 1, "Exactly 1 theme should be selected");
+  if (activeThemes.length > 0) {
+    is(activeThemes[0].theme.id, LIGHT_THEME_ID, "Light theme should be selected");
+  }
+
   let firstLWTheme = recommendedHeader.nextSibling;
   let firstLWThemeId = firstLWTheme.theme.id;
-  let themeChangedPromise = promiseObserverNotified("lightweight-theme-changed");
+  themeChangedPromise = promiseObserverNotified("lightweight-theme-changed");
   firstLWTheme.doCommand();
   info("Clicked on first theme");
   await themeChangedPromise;
 
   popupShownPromise = popupShown(popup);
   EventUtils.synthesizeMouseAtCenter(themesButton, {});
   info("Clicked on themes button");
   await popupShownPromise;
 
+  activeThemes = popup.querySelectorAll("toolbarbutton.customization-lwtheme-menu-theme[active]");
+  is(activeThemes.length, 1, "Exactly 1 theme should be selected");
+  if (activeThemes.length > 0) {
+    is(activeThemes[0].theme.id, firstLWThemeId, "First theme should be selected");
+  }
+
   is(header.nextSibling.theme.id, DEFAULT_THEME_ID, "The first theme should be the Default theme");
   let installedThemeId = header.nextSibling.nextSibling.nextSibling.nextSibling.theme.id;
   ok(installedThemeId.startsWith(firstLWThemeId),
      "The second theme in the 'My Themes' section should be the newly installed theme: " +
      "Installed theme id: " + installedThemeId + "; First theme ID: " + firstLWThemeId);
   let themeCount = 0;
   let iterNode = header;
   while (iterNode.nextSibling && iterNode.nextSibling.theme) {
@@ -73,17 +95,17 @@ add_task(async function() {
 
   let defaultTheme = header.nextSibling;
   defaultTheme.doCommand();
   is(Services.prefs.getCharPref("lightweightThemes.selectedThemeID"), "", "No lwtheme should be selected");
 
   // ensure current theme isn't set to "Default"
   popupShownPromise = popupShown(popup);
   EventUtils.synthesizeMouseAtCenter(themesButton, {});
-  info("Clicked on themes button a second time");
+  info("Clicked on themes button a fourth time");
   await popupShownPromise;
 
   firstLWTheme = recommendedHeader.nextSibling;
   themeChangedPromise = promiseObserverNotified("lightweight-theme-changed");
   firstLWTheme.doCommand();
   info("Clicked on first theme again");
   await themeChangedPromise;
 
@@ -93,17 +115,17 @@ add_task(async function() {
 
   await endCustomizing();
   Services.prefs.setCharPref("lightweightThemes.usedThemes", "[]");
   Services.prefs.setCharPref("lightweightThemes.recommendedThemes", "[]");
   info("Removed all recommended themes");
   await startCustomizing();
   popupShownPromise = popupShown(popup);
   EventUtils.synthesizeMouseAtCenter(themesButton, {});
-  info("Clicked on themes button a second time");
+  info("Clicked on themes button a fifth time");
   await popupShownPromise;
   header = document.getElementById("customization-lwtheme-menu-header");
   is(header.hidden, false, "Header should never be hidden");
   let themeNode = header.nextSibling;
   is(themeNode.theme.id, DEFAULT_THEME_ID, "The first theme should be the Default theme");
   is(themeNode.hidden, false, "The default theme should never be hidden");
 
   themeNode = themeNode.nextSibling;