Great feedback, thank you! I've never done anything with sound yet, but I'll try to figure it out before next time.
You can get on platforms by double jumping, I thought it'd be good to have it that way so that you always use your first jump to dodge projectiles without accidentally ending up in other platforms. That doesn't really come up in the demo though because there's only one type of projectile and it mostly moves vertically.
Yeah the visuals need a bit of an overhaul. I've always been bad at picking colors and I've found it pretty hard to make the important parts stand out while keeping everything visually coherent.