diff --git a/src/ZenThemesCommon.mjs b/src/ZenThemesCommon.mjs
index 6e2b4ce..b59ef92 100644
--- a/src/ZenThemesCommon.mjs
+++ b/src/ZenThemesCommon.mjs
@@ -12,6 +12,14 @@ var ZenThemesCommon = {
     return this.kZenOSToSmallName[os];
   },
 
+  get browsers() {
+    return Services.wm.getEnumerator('navigator:browser');
+  },
+
+  get currentBrowser() {
+    return Services.wm.getMostRecentWindow('navigator:browser');
+  },
+
   get themesRootPath() {
     return PathUtils.join(PathUtils.profileDir, 'chrome', 'zen-themes');
   },
@@ -24,22 +32,15 @@ var ZenThemesCommon = {
     return PathUtils.join(this.themesRootPath, themeId);
   },
 
-  getBrowser() {
-    if (!this.__browser) {
-      this.__browser = Services.wm.getMostRecentWindow('navigator:browser');
-    }
-
-    return this.__browser;
-  },
-
   async getThemes() {
-    if (!this._themes) {
+    if (!this.themes) {
       if (!(await IOUtils.exists(this.themesDataFile))) {
         await IOUtils.writeJSON(this.themesDataFile, {});
       }
-      this._themes = await IOUtils.readJSON(this.themesDataFile);
+
+      this.themes = await IOUtils.readJSON(this.themesDataFile);
     }
-    return this._themes;
+    return this.themes;
   },
 
   async getThemePreferences(theme) {
diff --git a/src/ZenThemesImporter.mjs b/src/ZenThemesImporter.mjs
index 2e2d357..7302f34 100644
--- a/src/ZenThemesImporter.mjs
+++ b/src/ZenThemesImporter.mjs
@@ -12,14 +12,18 @@ const kenStylesheetFooter = `
 var gZenStylesheetManager = {
   async writeStylesheet(path, themes) {
     let content = kZenStylesheetThemeHeader;
+
     for (let theme of themes) {
       if (theme.enabled !== undefined && !theme.enabled) {
         continue;
       }
       content += this.getThemeCSS(theme);
     }
+
     content += kenStylesheetFooter;
-    let buffer = new TextEncoder().encode(content);
+
+    const buffer = new TextEncoder().encode(content);
+
     await IOUtils.write(path, buffer);
   },
 
@@ -76,7 +80,7 @@ var gZenThemeImporter = new (class {
   }
 
   async rebuildThemeStylesheet() {
-    this._themes = null;
+    ZenThemesCommon.themes = null;
     await this.updateStylesheet();
   }
 
@@ -168,7 +172,7 @@ var gZenThemeImporter = new (class {
   }
 
   writeToDom(themesWithPreferences) {
-    const browser = ZenThemesCommon.getBrowser();
+    const browser = ZenThemesCommon.currentBrowser;
 
     for (const { enabled, preferences, name } of themesWithPreferences) {
       const sanitizedName = `theme-${name?.replaceAll(/\s/g, '-')?.replaceAll(/[^A-z_-]+/g, '')}`;
@@ -183,9 +187,7 @@ var gZenThemeImporter = new (class {
         for (const { property } of preferences.filter(({ type }) => type !== 'checkbox')) {
           const sanitizedProperty = property?.replaceAll(/\./g, '-');
 
-          if (document.querySelector(':root').style.hasProperty(`--${sanitizedProperty}`)) {
-            document.querySelector(':root').style.removeProperty(`--${sanitizedProperty}`);
-          }
+          browser.document.querySelector(':root').style.removeProperty(`--${sanitizedProperty}`);
         }
 
         continue;
@@ -216,9 +218,9 @@ var gZenThemeImporter = new (class {
 
           case 'string': {
             if (value === '') {
-              document.querySelector(':root').style.removeProperty(`--${sanitizedProperty}`);
+              browser.document.querySelector(':root').style.removeProperty(`--${sanitizedProperty}`);
             } else {
-              document.querySelector(':root').style.setProperty(`--${sanitizedProperty}`, value);
+              browser.document.querySelector(':root').style.setProperty(`--${sanitizedProperty}`, value);
             }
             break;
           }
@@ -232,7 +234,7 @@ var gZenThemeImporter = new (class {
 
   async writeStylesheet(themeList) {
     const themes = [];
-    this._themes = null;
+    ZenThemesCommon.themes = null;
 
     for (let theme of themeList) {
       theme._chromeURL = this.getStylesheetURIForTheme(theme).spec;