[Theme Submission] Add XP Balloon theme.

Adds a new theme, finally. I'm not sure if this name will suffice, but it is short and accurate.

Resolves #139 (closed).

I used the existing Retro theme until it bugged me enough to make this.

Since there haven't actually been any theme submissions prior to this, and there isn't a template for it, I'm not entirely sure what else to put here besides the theme and some screenshots: