Style looks good but I have some suggestions:

• #box relies on a horizontal scrollbar, this can be annoying so might I suggest some adding simple JS to make it scrollable via dragging left or right?

• The height of #container should be auto instead of a fixed size. You can add some padding-top and padding-bottom to make it look better if setting size to auto ruins it for you.

• You used a fixed width in #container of 50%, when I started making websites I did that too, but I would suggest making it dynamic and mobile friendly using min(). Perhaps setting height to min(50em,100%) would work for you? As a bonus this means the size of your header is different from the page which makes it more interesting.

• Css variables. Google them. It will change everything.