you can more or less easily apply a theme to miranda, if you have downloaded the necessary files (depending on theme/miranda version/plugin version).
the process to create a new theme is a little confusing, i didn't figure it out to be honest. i copied a downloaded skin and changed the images with mine and renamed a few things, that's it.
what you need to do in order to create a theme also depends on the plugin you use, i guess the most common theme plugin is Contact List Modern (google it, you'll find plenty). well, i'm afraid i can't tell you more, maybe some other helpful people post something you (and I) can use here, too