Currently only one custom theme may be uploaded per site, saved with the name 'custom' and stored into the local/themes subdirectory.
Administrators can upload a .ZIP archive containing a theme through the design admin panel; its contents are validated to ensure that only legit files are saved, and a 5M size quota is enforced.
Theme upload requires the zip extension for PHP; if not present, theme uploading is disabled by default.
Uploading and the custom CSS can be controlled via $config['theme_upload']['enabled'] and $config['custom_css']['enabled'].
Configurable directory/path/server for 'local' subdirectory (currently only as used for themes; local plugins not yet switched over)
Can set $config['local']['dir'] etc; not currently exposed in the admin panels.
Per-site directories on a separate themes server could be set up such as:
$config['local']['dir'] = '/path/to/themes/local/' . $_nickname;
$config['local']['server'] = 'themes.example.com';
$config['local']['path'] = '/local/' . $_nickname;
$config['local']['ssl'] = 'never';
