feat(zellij): make top and bottom bars transparent (folke#587)

## Description

This PR expects a user to use the Tokyonight theme for the running
Terminal too.
The black color in Zellij should match the Teriminal background color,
This ensures the top and bottom bars are transparent

## Related Issue(s)

No

## Screenshots

- Before

![image](https://github.com/folke/tokyonight.nvim/assets/17734314/70105867-7d19-47da-b2fe-f78a5374e6d5)

- After

![image](https://github.com/folke/tokyonight.nvim/assets/17734314/24aa2c78-89ec-4544-be93-3d6fa04decea)

lua/tokyonight/extra/zellij.lua

@@ -14,7 +14,9 @@ themes {
     ${_name} {
         fg "${fg}"
         bg "${bg_highlight}"
-        black "${black}"
+        // Black should match the terminal background color
+        // This ensures the top and bottom bars are transparent
+        black "${bg}"
         red "${red}"
         green "${green}"
         yellow "${yellow}"