A WordPress child theme extends a WordPress parent theme by adding styles and functions that work with or override the styles and functions of the parent.
Both the child theme and its designated parent theme must be loaded however only the child theme is activated.
Why do we use child themes?
Using a child theme for theme modifications allows site owners to update the parent theme without losing those modifications.
Themes get updated frequently – security updates, bug fixes, improvements, etc… . If theme files have been edited those edits will be overwritten when the theme updates. We have the “Additional CSS” block in the WordPress Customizer but sometimes you need a LOT of css.
Lets say we want to use the TwentySixteen theme. It’s well built and perfect for the project except it has that dorky 20 pixel black border around the whole site.
Editing the CSS file of the TwentySixteen theme to remove that border is simple.
However if we edit the TwentySixteen stylesheet directly the next time it is updated all our edits will be over-written and the black border will return.
Therefore, rather than editing the TwentySixteen theme file we make a child theme and add the CSS to override the black border there.
Using a child theme we can confidently update the parent theme knowing that we will not be overwriting our customizations – the border will stay away AND the site will get the benefits of whatever updates are made to the parent theme.
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