With WebSelf, you have a basic 404 Error page if one of your pages is not found.
However, you have the option to customize it, so you will need to upgrade to the Growth plan.
You can then modify your 404 page and match it with the colors of your site.
Your 404 page is located with all the other pages of your site. So, click "Pages" at the top left of your screen, then click "Add a 404 page."
Once you have upgraded your plan, the 404 page will appear like this on your screen, and you can then edit it just like any other page by adding a section or text.