I'd like to change some of the styles in the admin interface without editing files in cms/ or sapphire/ (in order to not break on update). Is this possible? It would be nice to have a system in place for admin themes, so people could develop their own themes and switch them out as desired. Will this be part of v3?
In 2.4 you can alter the styles from your theme with a css file called 'editor.css' this will be loaded automatically in the CMS.
In 3.0 we're introducing the concept of a CSS 'theme' for the backend, similar to a jQueryUI theme. You'll be able to alter colors and fonts but layout / etc will be a little harder to edit. You can see an example of this as a work in progress at - https://github.com/silverstripe/sapphire/blob/master/admin/scss/themes/_default.scss