In macOS Tahoe Beta 2, Apple included a new option to add a background to the menu bar, making it possible to have a menu bar ...

2. Give the folder a name, such as Icon Tutorial. 3. ... After closing the window, you’ll see that your new icon has a transparent background with clean, smooth edges.

When you try to save such an image, the background area of the image will no longer be transparent and turns into solid color; in most cases, white i.e. Color 2.

You don't want to space icons out with little generic Windows executable icons after all, you want to space them out with nice transparent open space. We've created a 256x256 transparent icon for you.