theme: Update borderBar to follow change in Figma design

See also:
  #928 (comment)

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/widgets/theme.dart

@@ -117,7 +117,7 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
       bgContextMenu: const Color(0xfff2f2f2),
       bgCounterUnread: const Color(0xff666699).withValues(alpha: 0.15),
       bgTopBar: const Color(0xfff5f5f5),
-      borderBar: const Color(0x33000000),
+      borderBar: Colors.black.withValues(alpha: 0.2),
       contextMenuCancelText: const Color(0xff222222),
       contextMenuItemBg: const Color(0xff6159e1),
       contextMenuItemText: const Color(0xff381da7),
@@ -153,7 +153,7 @@ class DesignVariables extends ThemeExtension<DesignVariables> {
       bgContextMenu: const Color(0xff262626),
       bgCounterUnread: const Color(0xff666699).withValues(alpha: 0.37),
       bgTopBar: const Color(0xff242424),
-      borderBar: Colors.black.withValues(alpha: 0.41),
+      borderBar: Colors.black.withValues(alpha: 0.5),
       contextMenuCancelText: const Color(0xffffffff).withValues(alpha: 0.75),
       contextMenuItemBg: const Color(0xff7977fe),
       contextMenuItemText: const Color(0xff9398fd),