Add a 525 x 450 pixel rectangle for a background. Select the Fill Tool
(F5). From the Fill Type drop down list, select Fractal Plasma. Use the
two colors shown and select the Alt Rainbow fill effect. Drag the end of
the horizontal arrow and rotate the angle of the fill upwards to about 15
degrees.
Click on the fill again to enable the Fractal Resolution and Fractal Scale
text entry boxes (on the Infobar). Change the two values as shown top
right. Press Enter to apply each change. Open the Profile dialog and
edit the slider settings as shown.