Base Style: paulwasit Habitica
Task Bars: Dark Energy - HabitRPG/Habitica
Background for self and party members: HabitRPG / Habitica, little UI tweaks
Character and pet animations and backup background image: merfy's HabitRPG — Custom Compact Edition

These guys are awesome, and deserve all the credit! They're the first few results when you look for a Habit theme, so go check 'em out!

Update #1 - 11/10/15:
Added a backup background image, for when the main one stops working. You have to go into the style settings and replace the image link to change the image, I don't know how to make the backup automatically replace the main one.

Update #2 - 3/21/16:
Changed the theme to incorporate some of the more visually pleasing aspects of paulwasit's theme, such as centering the sections in each category, hiding the borders of the top option buttons and making the body of each task white, rather than its color based on task value.