Why purchase a custom theme?
It means that, your website is the only one that uses it, and does not resemble any of the other tens of them that are out there that use the same theme with a different color scheme.
The theme can be an exact implementation of your design, right down to the pixel. Rather than having to settle for somebody’s design decisions and be limited by the customization and features built into the theme, you can create something that is exactly what you designed.
The custom theme is less likely to have major security holes, just because of the fact that you are using fewer features and less code if not anything else. Even though, it is probably next to impossible to know of sure about bugs, you are not waiting for a security or bug patch from a third-party vendor. You can fix them as you find them.
Search Engine Optimization (SEO)
A custom theme is the best suited for SEO. A custom theme is usually very lean with almost no extra source code. There are no extra features that are not required and the required features can be built into the theme itself rather than having to use extra plugins (of course depending on the feature). Furthermore, you can use the appropriate HTML tags (like h1, h2, h3 etc) for your content without having to over customize layouts.
The biggest advantage in SEO here is the lean coding which makes the websites very light. There are several frameworks and themes that specializes in speed which are always an option but a custom theme is the best option regardless.
No Feature Overload
Most third party themes have a large amount of features and customization code. These are required to so that it can cater to several different clients with different requirements. Custom themes does not need a plethora of features that you would never use and don’t need. All of these unwanted features adds a lot of executable code and can potentially slow down your website.
Limit Plugin Usage
You can built the functionality of most of the plugins that you use straight into the theme. This is especially true for the plugins that you are used for content layout and customization. This can reduce the dependency on third party plugin and make the theme faster, as now the plugin code does not have to be injected at run time.
Not all plugins can be built in though, depending on the functionality that it provides.
Small Static Files
You do not need to have tons of code, which makes every piece of content customizable because it is already customized for your design in a custom theme. All of these customizing code can add to the weight of your page, and make it slower especially when it is not coded correctly.
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