Individual tiles and sprites within individual tilesets and levels may be 32-bit. This means that they don't depend on the current level (or tileset)'s 256-color palette, and that they may use an alpha channel.

They are rendered in a 16-bit window.

The example level has many examples of 32-bit graphics and lots of text explaining which ones are 8-bit (the ones that turn red) and which ones are 32-bit (the ones that don't) and why.