What do the standby background, screensaver background and menu background in the theme of mobile phone mean?

Hello! Let me tell you this: changing a theme is just like changing from formal clothes (suits, pants and shoes) to casual clothes (jackets, jeans and sneakers), but the appearance changes more. The standby screen refers to the mobile phone desktop, and the menu refers to the function items (such as information, address book, etc.). ), and the screen saver refers to the screen saver of the mobile phone, which generally refers to the display of the screen when the mobile phone is not operated within a set time, mostly for power saving. I hope you can understand this and help you.