Wow ! Thank you so much for this wonderful comment !
Indeed, there's a CRT effect on the background and I also used RGB shift for the level selection menu.
As for the rest, it's all about lighting ( by the way, creating the black light effect was quite tricky, since Gdevelop assumed it was a lack of light by default ) and particles ( tons of them )...
PS : I imagine you didn't encounter any menu bugs, but just in case you did, feel free to install the ".exe" ( check the link in the game page description ) to enjoy it even more !