How to create a custom Admin skin for WP that also works with multisite

Hi

How can I create a custom Admin theme for Wordpress starting from a HTML template.

Also how do I ensure that it works with future upgrades to wordpress- how do I create a plugin that specifically tests my custom Admin skin with a newly released WP version

(3) How do I set it as default admin theme which child site users in WPMU cannot change? And that remains the admin theme even after WP upgrades to next version?

(4) How do I create a plugin to set a WP Admin theme created by me as the default admin theme for all child sites--> that child site users cannot change themselves? And that remains admin theme even after WP upgrades to next version?

Yours sincerely,
Arvind.