Well, if you release for web browser version of the game, you shouldn't use Ctrl key at all, because the default key of Ctrl + W for web browsers is closing current tab. Therefore, everytime players want to sprinting upward movement will be always closing the tab they are currently playing.
Although I don't see there is anything to do with the theme, the background for credits, main menu and the gameplay also don't seemed to be related to each other.
However, I will say this is nice platformer to play, you can freely move upward or downward, left and right, for shooting with the moving left or right will stuck and continue the moving though... Fortunately, this is okay for me as the main character hp is tough enough to take most of the attacks without worries.
Also the ramming damage is based on your current base damage with current size, is quite fun to do! Although enough current damage is more than enough to kill off every normal enemy, the boss will always be stunned should you always spam shooting key.