Combine common CSS into one file - otherwise you'll have to make adjustments in all of them and make mistakes
Test all pages on different window sizes (resize window and see how pages react)
Make an object of ships which will include ship id, ship image path, ship name, ship hp, and any parameters you need. The same you did with intro pages.
Make const objects for references instead of using numbers. For example:
const REWARDS = {
START : {
name : 'Basic ship',
unlockShip : SHIPS.BASIC,
}
}
Notes:
const SHIPS : { BASIC : 1, INSECT : 2, }