Customisation adapts a website’s settings, presentation or behaviour to a project’s needs. In Joomla, this can range from selecting a template colour to developing a dedicated extension.
Available approaches
Start with Joomla, template and extension settings for changes supported by their developers. Custom CSS and JavaScript can refine presentation and behaviour when loaded through supported mechanisms.
A layout override changes a component’s or module’s HTML output. A child template separates some custom changes from its parent. New business logic may require a plugin, module or component rather than CSS alone.
Surviving updates
Editing Joomla core or third-party extension files directly can lose changes during an update. Use settings, overrides and documented extension points. Overrides still need maintenance because new releases may change data or markup requirements.
Checking changes
Work on a separate copy and preserve the original state. Check related pages, mobile layouts, keyboard use, forms and both language versions. Scope styles and scripts to the intended area to avoid affecting other Joomla sections.
Record what changed and how to restore the previous state. Consider update and support costs before adopting a custom solution.
Summary
Joomla customisation should meet project needs and remain maintainable. Choose the appropriate modification level and recheck it after updates.