How to Allow Users to Customize Page/Upload their Own HTML/CSS

I’m relatively new to webDev, but I’ve always wanted to build a website that allows users to customize their own page design (the same way tumblr would allow you to upload your own HTML, or landing page builders allow you to customize the shape of your page).

I’ve searched up about this a bit, but I’m not sure whether I’m not asking the right question, or if it’s not easily available to find, since the only answers I could find were usually people already implementing this and asking about security questions (relating to JS and injection attacks). My question is more along the lines of where do I start, what should I read/learn to be able to do this.

Any help is appreciated!