Each theme consists of a single PNG image file. This image consists of an array of seven rows, corresponding to the different object colors/shapes, and four columns, each of which is a frame in the animation of the object. The actual size of the image is irrelevant, it will be rescaled, but the sub-images must form a four by seven grid to get the correct effect. When using SVG remember that the image size is the page size, if your grid does not fill the page try shrinking the page size. Use an existing theme as a guide.