The game looks, feels and sounds very polished.
First of all, the colour palette feels very retro-like and fits the gameplay well. Enemies have anticipation animations, which I've never really seen for this kind of game. Projectiles and shields also have varying sprites depending on level, which is nice. The only thing that feels slightly off is that the boss' projectile disappears immediately after hitting the player.
Gameplay-wise, it's a straightforward shooter. Instructions are displayed clearly, UI is very simple and clean. Having a leaderboard is a fitting addition to making it feel even more retro-like. Making the upgrades affect the shot pattern is also pretty cool. I personally think that the hitbox of the player is a little too big, making it difficult to dodge projectile sometimes, but that's just to nitpick.
As for audio, both the music and sound effects fit the aesthetic well. There is proper feedback on pretty much everything that is happening on screen. Not sure if I'm imagining things, but I think the transition from normal theme to boss theme is also nicely done. One thing that I noticed is that there is no death SFX for enemies. Having that might make it more satisfying when killing them.
Definitely a fun game. I would love to know how this relates to the theme. Still, great job!