Here are some amazing website sections you can make using flexbox layouts, and the basic steps to create theme.
Example A
The main container has horizontal alignment with two flex groups.
The left flex group is horizontal with wrapping and spacing, each of the child groups has a fixed width.
The right frame has two groups inside, with vertical direction and space between - so one is at the very top and the other at the bottom.
Example B
Example C
Example D