WordPress is the most widely used platform for building sites of all types. It can be complex for the user but can accommodate a wide range of projects.
Even simple themes take time and are complex to implement. They do not make twenty part tutorial series on theme development because it is easy. You can not just graft on your old HTML and CSS files like some seem to think. It is far more complicated than that.
I have a sandbox site that I use for theme development. Here you can see any theme that is being worked on at the moment.
A typical wireframe is shown below. This is the first step in designing a theme.The layout here is just for the homepage, there are many other layouts in a theme.
In WordPress themes, separate layouts are needed for the below page and post types.
Next is a simplified guide that should give you an idea about the amount of time and work involved in a "simple" theme.
The header usually contains the main menu.
Widget areas are used to add extra items such as the search bar, archives and many others. Once enabled each widget area will appear in the admin section.
In WordPress a page named page-home.php is used to style the home page only. Here the default Header and Footer files are included.
The single page is used for a single blog post or page it had a different custom layout.
If you look on the left hand side of the WordPress admin section you will see a section named Format. Each of these formats has a separate file that has to be designed.
Break points for mobile
You do not need to know the above in order to commission a project, it is only to explain why most themes or conversions from PSD files cost what they do!
It will take three to four days, at least to create a fully functioning theme.