Skinner is a real monster of an addon - if you're looking to reskin just the Character Pane, I'd recommend finding an alternative that does just that so you can save on addon memory.

Most custom UI packs I've seen that modify the vanilla interface's art do so by overriding the image files in WoW's .mpq libraries with their own art. Now this isn't at all like the model replacement hacks that Blizzard's broken in the past - you're not modifying the .mpq files or adding ones of your own. Instead, if you mirror the file path of the image you're trying to replace (typically within a folder in the Interface directory), when WoW loads up, it'll use your images over the ones stored in a .mpq. You can replace fonts this way too.

Example:

Say you wanted to change default UI buttons from red to a different color. Creating a Buttons directory within your Interface folder (WoWDir/Interface/Buttons) and generating new .blp image files with the same name as those you wanted to replace (in this case UI-Panel-Button-Down.blp, UI-Panel-Button-Highlight.blp, and UI-Panel-Button-Up.blp) would let you do just that. It also has the practical upshot of not costing you any addon memory or CPU cycles while playing, as there's no legwork done by addons to replace textures during loadup and no ingame configuration needed.