Settings / Themes / Learn how to / Customize themes points to a Not Found page.
Thanks. This should resolve during the next automatic rebuild of the Guides web site. The link in the program is valid, and I just verified the source article is valid. So I think it was a problem with execution of the automatic process that scans for new Guides, updates the web site, and writes the list of links into the program.
If you need the article in the meanwhile, it’s on the Forum at https://forum.manager.io/t/customize-themes/10368.
This link is now functional. Because of the way guides are incorporated into the program, you may need to update to the latest version before it will work correctly. The delay was because the web site was under revision.