Modding is the act of modifying game files (usually by just replacing them) to change things about it. It's pretty easy to mod SK, specially the textures since they're in a .png format.

You sorta have a point with the gorgos though, I don't really like their look. Their coloring should resemble more with their devilite counterparts, and the spikes on their backs are kinda lame.

To add up, I think the artist is making the armors' legs too dark recently. Look at the warden costume's armor for example. The color of the legs is far darker than the half top of the body.

Textures require no coding at all.
For the sake of modding in the sense of just modifying existing texture content, you would just edit the proper 'image' file (not necessarily for SK, this is generic). It is technically against ToS, but as long as you don't post images showing it off, you can't be stopped (although I've seen many people post images that accidentally show off modified text fonts or textures, and not get banned.... OOO seems pretty lax if it doesn't make it seem like you're "cheating")