WordPress web designers often use WordPress themes to create websites, but this does not mean that it is merely a matter of installing the theme, plugging in content, and you are done.
Even if a WordPress theme is used during web development, there are so many things that have to be customized before the website looks and functions the way you want it to. Some people have the notion that their website will look exactly like the demo theme that is presented by the company or developer site selling the theme that is purchased. This is not the case. Once the theme is installed, it looks nothing like the demo versions of the website, and many novice users are disappointed and frustrated by this, and they have to consult or hire a WordPress web developer.
WordPress plugins are very similar in this regard. Many plugins that are created by third party programmers do not function well, or the plugins do not match the design of your WordPress website. In these cases, you must customize the PHP and CSS code within the plugin files to obtain the look and feel you are after.
Other crucial skills that are necessary when designing and developing a WordPress website is the expert knowledge of web graphics creation software such as Adobe Illustrator and Adobe Photoshop. Without graphic and web design skills in the Adobe Creative Suite, you are basically stuck with uploading photos and graphics that are not original, and not sized or treated properly. The end result is a very amateur and unattractive looking website.
Basically, WordPress themes are handy to start any web design and development project, but if you want a professional looking website, they really should be handled by a professional WordPress web designer.