If you use an off-the-shelf theme and need to customise it, it’s crucial to create a child theme first.
Child themes are clones of the main site theme where you can edit template files (code).
When your main theme is updated, nothing is the child theme is changed, so you don’t overwrite customisations to your site.
While child themes are great for free or paid-for themes, we prefer to create bespoke themes which don’t require a child element. It makes things simpler, quicker and in most cases, more cost-effective.
Many off-the-shelf themes can be hugely complicated, so unless you are experienced in coding, be careful when using child themes.