chore: Update Zen Browser styles for dark mode compatibility

src/browser/base/content/ZenUIManager.mjs

@@ -59,5 +59,9 @@ var gZenVerticalTabsManager = {
   toggleExpand() {
     let expanded = !this.expanded;
     Services.prefs.setBoolPref('zen.view.sidebar-expanded', expanded);
+
+    let toolbox = document.getElementById('navigator-toolbox');
+    toolbox.removeAttribute('width');
+    toolbox.style.removeProperty('width');
   },
 };

src/browser/base/content/zenThemeModifier.js

@@ -90,6 +90,15 @@ var ZenThemeModifier = {
       }
     }
     appWrapepr.setAttribute("hidden", "true");
+
+    // Set a splitter to navigator-toolbox
+    const splitter = document.createXULElement("splitter");
+    splitter.setAttribute("id", "zen-sidebar-splitter");
+    splitter.setAttribute("orient", "horizontal");
+    splitter.setAttribute("resizebefore", "sibling");
+    splitter.setAttribute("resizeafter", "none");
+    const titlebar = document.getElementById("navigator-toolbox");
+    titlebar.insertAdjacentElement("afterend", splitter);
   },
 
   _zenInitBrowserLayout() {

src/browser/themes/shared/zen-browser-shared.css

@@ -617,13 +617,26 @@ panelmultiview {
 }
 
 /* Expanded sidebar */
+#zen-sidebar-splitter {
+  display: none;
+  opacity: 0;
+}
 
 @media (-moz-bool-pref: "zen.view.sidebar-expanded") {
   #navigator-toolbox {
-    min-width: 170px;
+    min-width: 150px;
     align-items: start;
     padding-left: 10px;
     padding-right: 10px;
+    max-width: 210px;
+    transition: .2s;
+    width: 170px;
+    border: none;
+  }
+
+  #zen-sidebar-splitter {
+    display: block;
+    width: 1px;
   }
 
   .tab-label-container {