• To begin with I opened a new canvas of size 900x900
• Drew a vector square covering the complete canvas
• Provided gradient radial fill in shades of blue as shown below
  
  
• Selected vector tool from the tool box, selected oil splatter stroke and drew a line on the canvas
  
  
• To customize this oil splatter stroke, I entered into ‘stroke options’
  
• Saved the new stroke as 'splatter2'
  
  
• Changes I made there is as shown below
  
  
  
  
• To this stroke I give an inner glow filter in white.
• Changed blend mode to ‘Red’ in layers window.
  
  
• Your effect is now ready.

Wasn’t it simple! :)

Tags: Fireworks custom stroke,  Fireworks custom oil stroke, Fireworks background