It's a good idea to approach theme design from two angles:
There are many benefits to using or tweaking off-the-shelf themes. First, you save a lot of time getting your site up with a nice theme design. Second, you don't need to know as much about CSS, HTML, or PHP. This means that with a little web surfing, you can have your WordPress site up and running with a stylish look in no time at all.
Using an off-the-shelf theme is a great way to get started with WordPress. It means you can build your first WordPress site with a coding experience, or with experience of writing code but not of building themes. You can also work with an existing theme to create your own, either editing the theme directly or by using a child theme (which we'll come to later).
But if you need to build a bespoke site, using an off-the-shelf theme will have its drawbacks and may not save you as much time as you would hope for.
Perhaps your site needs a special third-party plugin for a specific type of content; it might not look quite right without a lot of tweaking. And while we're discussing tweaking, every CSS designer is different and sets up their theme's template files and stylesheets accordingly. While it makes perfect sense to them, it can be confusing and time consuming to work through.
Working with an off-the-shelf theme starts off feeling like the simplest approach, but as you delve deeper into the code, you may find yourself making more and more changes until you reach the point where it would have been easier to start form scratch.
Before making use of an existing theme, check if it really has everything you'll need for your project with minimal tweaking. And check its license—it should be GPL, like WordPress itself.