You have there several possibilities to select canvas padding color. Canvas is the surface the image lays on: it looks like a frame around the image in the image window. That's only a matter of personal reason as this option doesn't interfere with the image itself. Don't mistake this color for the color used by the Fill tool.
That's theme defined in Preferences/Theme
Ckeck representing transparency is defined in Preferences/Display.
This button allows you to return to your choice in Image Window/Appearance.