how to make seprate themes for your main page?

[EdgyRabbid]

So for the entire time ive been making my website, its been rayman themed.

However, ive been hyperfocused on club penguin, and wanted to make another theme for it. Can someone tell me how i could do this? Would it require Java? I just want a function that would switch the css and images on my website.

[grilledmint]

On my site I have theme switcher that actually don't use js! But it is using for "save" function so people can save info in their browser about their choice
This tutorial that help me a lot: https://solaria.neocities.org/guides/themeswitch