Nice game! I liked the main menu music, very chill :)
The game is addictive, even when for me was kind of difficult at the beginning. The aditional themes in the Zen Mode may have more differences from the default (particles, combo sounds)
[Bug?]
I pressed "Z" (several times) while the "Options Menu" was open, then closed the Menu and it got locked (I couldn't open it with the mouse left click) . Also, "Z" key seems to cache the last option I opened, eg, if i open "How to play" then close it and press "Z", this menu opens up again.