To create your own admin theme you need to do at least these things:
Create a folder in 'themes/admin', e.g. 'themes/admin/mytheme'.
Define inside your theme's folder at least these templates in it: header.tpl, footer.tpl, admin.tpl, message.tpl. You can use standard ones from 'system/admin/tpl' folder as example.
Define inside your theme's folder other admin templates found in 'system/admin/tpl' if necessary.