The start menu is not called "Change Wallpaper", it is called "Change Background Image". You really need to change the theme, and many good-looking themes are anime-type cute themes. For example, in addition to the background image in the picture below, there are also "irregular hollow start menu" and "fully transparent start menu". These are relatively rare in ordinary themes.

You don’t need a Rubik’s Cube to change the theme, just do it manually. As for how to modify an ordinary theme into a theme like the picture below, Rubik's Cube is also unable to handle it and requires specialized theme creation software.