Hi Bodhi,
You can try using a file manager such as Astro or Root Manager to look at the sd card. Open the sd card and look for MIUI. Open that and then open the folder called "theme". In that folder you will find the themes that are installed. Open the theme that you want to modify and look at the framework/res.
What I normally do is copy the theme to my desktop and unzip it there. I modify what I want, zip it back up. Then I copy it back to my sd card, replacing the previous version and close everything up. Re-apply the theme in the theme manager and you are set. There might be an easier way but that is how I do it.