I liked the old default theme, but new choices were quite interesting. While most themes look nice, I had some complaints about them. Some color themes were too saturated and most themes had a very poor contrast between read and unread emails. The theme that I liked the most was Phantasea because its header and color changes throughout the day relative to the position of the sun while preserving good contrast between new and old messages.
To change your theme go to Settings (upper right corner) > Themes
Source: Spice up your inbox with colors and themes (Google Blog)