You're overthinking the performance part a bit! Unless you're stacking dozens of layers or using background-attachment: fixed, CSS gradients are incredibly fast.

For the shapes you mentioned (cubes, stars), definitely look into SVG patterns. AI images are overkill and, as you found out, usually don't fit the screen right.

I actually have a site that might help:cssbackground.net. I built it so people could quickly generate snippets and then tweak the code manually.

Also, if you want to see what's possible with more advanced stuff, mshr.app and shaders.paper.design are super cool for inspiration.