A theme controls how your WordPress site looks and how its content is presented. It is often more than a skin: while many themes simply change the design, others, particularly those sold as premium themes, add significant functionality. All of them do this without modifying the WordPress core files, which is what makes WordPress upgrades possible.
Choosing a theme on appearance alone is the most common and most expensive mistake made when building a WordPress site. The demo always looks good. What matters is what happens after you have put your own content into it and lived with it for two years.
WordPress maintains its own free themes directory containing thousands of themes, all of which have been through a review process before listing. They also maintain a list of commercially supported theme developers.
The directory also includes the default themes that ship with WordPress itself. These are lightweight, well maintained and updated alongside core, and for a straightforward site they are a perfectly sensible choice.
Paid themes typically offer more functionality, more layout options and dedicated support. The trade off is that they tend to be heavier, and some are built around a proprietary page builder.
Buying from an established marketplace or directly from a developer with a visible update history is worth doing. Sales volume and review counts are a reasonable proxy for whether a theme will still be maintained in three years.
What is emphatically not worth doing is installing a “nulled” or cracked premium theme from a file sharing site. These very commonly contain injected code, and cleaning up afterwards costs far more than the licence would have.
Havenswift Hosting can build a custom WordPress theme to your specification, or build one to match an application you already run. If you have an e-commerce site and want to add a WordPress blog that matches it exactly, that is something we do regularly.
A custom theme makes sense when your requirements are specific enough that adapting an off the shelf theme would take as long, or when you want a lightweight theme carrying only the code your site actually uses. For many sites it is not necessary, and we will tell you when a well chosen existing theme would serve you better.
See also our page on WordPress plugins, and our WordPress hosting plans.