Use the mouse for the menus, and the keyboard in-game.
Move: Arrows / WASD
Shoot: Space / Ctrl
Pause: Q / Esc / Enter (opens the pause menu, where you can continue, retry or quit the current level)
Overlay on/off: O (scanline effect)"
If the game freezes, just click inside it (games can get unresponsive if you click elsewhere on the page).