Update demo app colors

[paulinea]

New tokens have been applied to the demo app designs in Figma and some colors have been changed:

• Cards background color in light mode (from black to light grey) and the associated content (from white to black)
• Token category headers background (from black to light grey) and the associated content (from white to black)
• Grid illustrations background in light mode (from black to light grey)
• Icon sizes illustrations background in light mode (from black to light grey)

Components illustrations have been changed to be displayed correctly on light surface in light mode.

All these changes must be applied to the demo app.