I'd like to ask the Omeka team to consider adding support for child themes. The suggested method of copying a theme folder wholly and making changes therein is neither a best practice nor a sustainable model. It keeps anyone who does so from being able to take advantage of any updates the original theme might release.
Omeka is already using the child themes model ... if a custom theme does not include a file, Omeka pulls the appropriate default file from the application/views/scripts folder. This is the same basic idea behind child themes, except the location of the default files would be specified in the child theme, and located in the themes folder instead of the application folder.
In addition to making Omeka more accessible to non-developers and those (like me) who don't have the time or resources to commit to full theme development, child themes could help spur theme development more generally. Taking responsibility for iterative changes is lower-stakes than for an entire theme.
There have been a couple of questions on the forums about this in the past (here and here), so there is definitely interest from the community. Please consider adding user-defined child themes to Omeka!