Honestly, my method is just brute forcing it. Do you have any experience with HTML and CSS? It's not necessary to already have any, I just figured things out by trying things out myself and googling things when I wanted to do something specific.
Like emokitty said, copying the code from an existing layout can be helpful. If you want to make your own, starting from an existing layout is useful to have an example you are building on top of.Â
My way of brute forcing it is like this:Â
When you're on your profile, press Shift + Ctrl + C at the same time and it'll open the console that allows you to see each element. There is also this helpful image:Â 
Basically each thing on the page is its own element, and if you want to change things such as the color of the background you apply that change to that element.Â
What I did when I was learning is that I would look at the elements and just mess around with things. Like for example on your profile, if I hover my mouse while in the elements console over this part, it tells me that the element is div.container. On the side it tells you the width of it and the margins. I used to just mess around with the numbers and such, so like changing the 810px to be 400px, or whatever - just to see in real-time what it does to the page.Â
Basically looking at an existing profile and seeing what changes can be made with the elements is helpful when thinking about what you actually want to do on your profile, like the background colors, the font, the margins, the borders.. a lot can be changed! If you want to change something in specific looking up, for example, "how to make borders rounded css" will often pull up a page from w3schools.com, which can be VERY helpful for when you're learning CSS (btw it brought me here:Â CSS Rounded Borders). If you have a question this website usually has examples for whatever you're trying to do.Â

The code itself is something you put in the about me section of your profile -- if done correctly, it won't show the code itself but will just apply it to the look of the profile.
I'm not sure if any of this is making sense tbh but if you want help with anything I don't mind trying to help, I kind of love messing around with things honestly. I could walkthrough my profile code w/ you or something ^^ Hope this helps somewhat! (????)
- Fen