WHMCS Child Theme Conversion

I currently have an existing WHMCS template that I would like to convert into a properly structured WHMCS Child Theme.

The primary objective is to preserve the existing design, branding, layout and customisations while ensuring that the template is protected from being overwritten when WHMCS or the underlying Nexus theme is upgraded.

The child theme should use Nexus as its parent theme and inherit any files, templates, layouts or functionality that have not been specifically customised. Where a file is required by WHMCS but does not exist within my child theme, it should automatically fall back to the corresponding file supplied by Nexus.

Required Structure

The finished implementation should follow WHMCS's supported child-theme architecture rather than creating an independent copy of the Nexus theme.

The child theme should contain only the files that need to be customised, for example:

theme.yaml
Custom CSS
Custom JavaScript where required
Custom header
Custom footer
Custom images/assets where required
Add aditianal .tpl files like hosting so that they can be developed in future.

Files that have not been modified should not be unnecessarily duplicated into the child theme.

Instead, the child theme should inherit them from Nexus.

Upgrade Protection

The most important requirement is that my customisations must remain separate from the parent Nexus theme.

When WHMCS, Nexus, or its associated template files are upgraded:

The Nexus parent theme can be updated normally.
My custom child-theme files remain untouched.
Any unchanged functionality continues to be inherited from Nexus.
If Nexus introduces improvements or fixes to files that I have not overridden, the child theme should automatically benefit from those updates.
My customised header, footer, styling and other overrides should not be overwritten.

This should provide a clean separation between the original Nexus theme and my HostZenix customisations.

Header and Footer

The existing template has customised branding and layout that I want to retain.

The child theme should therefore provide dedicated overrides for the header and footer, while continuing to inherit the remainder of the site from Nexus wherever possible.

This means the custom header/footer should be stored within the child theme rather than modifying the Nexus parent theme directly.

theme.yaml

A correctly configured theme.yaml file should identify Nexus as the parent theme and define the child theme appropriately.

The configuration should follow the official WHMCS child-theme methodology so that WHMCS understands the relationship between the child theme and Nexus.

Asset Management

Custom CSS, JavaScript, images and other assets should be stored within the child theme's own directory structure.

The implementation should avoid modifying Nexus's original assets wherever possible.

For example, if I need to change the appearance of the navigation, buttons, header or footer, those changes should be implemented through the child theme's CSS rather than editing Nexus's original CSS files.

Compatibility

The resulting child theme should remain compatible with the supported WHMCS version and should use the standard WHMCS template inheritance mechanism.

The conversion should also avoid unnecessary duplication of Nexus files, as maintaining a complete copy of the parent theme would defeat much of the purpose of using a child theme.

Desired Outcome

The final result should be an upload-ready WHMCS Child Theme based on my existing template and using Nexus as the parent theme.

The structure should effectively work as:

Nexus Theme
→ provides the standard WHMCS functionality and templates

HostZenix Child Theme
→ provides my branding, header, footer, CSS, assets and any other customised templates

This will allow Nexus to receive future updates without overwriting my customisations, while still allowing my child theme to inherit new files and improvements from Nexus automatically.

Overall Assessment

The existing template should therefore be converted rather than simply copied into a new theme.

The preferred approach is to identify exactly which files have been customised, move only those files into the child theme, configure theme.yaml to inherit from Nexus, and leave all unchanged functionality to the parent theme.

This will provide a much more maintainable WHMCS installation and significantly reduce the risk of losing customisations during future WHMCS or Nexus upgrades.