Improve PDF Admonition Blocks

[tajmone]

Admonition blocks could be graphically improved.

• Nicer Icons — This one seems tricky. I've managed to make the template load custom SVG icons from the document folder, but I can't find a way to load custom icons from the custom template folder. Must research this more.
• Colorize Admonitions by Types:
  • Icons colors according to type of admonition (red, yellow, green, blue, etc.).
  • Add Light BG colors?
  • Colorize border according to type.
  • Slighlty colored text according to type.
• Smaller font? (need to test this)