Builds repeating background patterns from pure CSS gradients — dots, lines, diagonal stripes, grid, cross, checkerboard, zigzag — or from inline SVG data URIs for waves and fractal noise, with pattern colour, background colour, tile size, thickness, opacity and angle. An optional gradient overlay is layered on top with its own opacity. Output is a CSS rule (background-color, layered background-image, size and position) plus a custom property; the preview renders it. Everything runs locally.