Also have a look at CustomizeMii and ShowMiiWads

ThemeMii is a manager for Wii Themes

It's currently able to:

• Load mym files and edit them
• Create new mym files
• Create a csm from any mym
• Create a mym from a csm
• Browse through base app's for path's (including TPL preview)
• Download base app's from NUS
• Automatically manage source's and image width and height