I was wondering which of the default Omeka themes that are available are most appropriate in terms of responsive design. It would be helpful if all of the themes incorporated some aspects of responsive design, like automatically resized layouts and menus. If I was to modify an existing theme to address responsive design, what is the best way of sharing this work?
Which version of Omeka are you working against? The themes shipped with Omeka 2.0 are both responsive and designed to work well on mobile screen sizes.
Admittedly, there are only two themes currently shipping with Omeka 2.0, but that number should grow fairly soon.
As for sharing any updates or modifications, that would depend on which theme you're updating, and who created it.
For any of the themes created by the Omeka team, we'd be happy to see any updates, reports, or modifications as pull requests or issues on the GitHub repository for each theme (for example, the Thanks, Roy theme's repository).
For themes written by other Omeka users, you should be able to check the "More Info" or "Get Support" links on the theme's page to get in touch with the theme's writer. Many user-contributed themes are also on GitHub or some other openly-available version control system.
I have one site that is 1.5.3 that is using the Berlin theme, and one that is 1.5.1 that is using the Seasons theme. It looks like Seasons and Thanks, Roy are updated for Omeka 2.0. I checked GitHub for the Berlin theme, and it looks like the developer is currently testing it for 2.0 and for mobile device display.