Ever since version 0.7.10 you have been able to replace the icons in calibre with icons of your own choosing. I looked around and found three partial or complete icon set replacements for calibre's default set. If anyone else has created a set I would love to know and add them to this list.
Personally I found the set for Ubuntu to be a nice change of pace. It did cause me to move the Search history and Quick preference plugins from the main toolbar to the optional text menu bar so the icons wouldn't clash.
Change your Calibre look and feel with Sphere 1.2 Calibre Set: Sphere 1.2 Calibre Set
Release: 0.7.10 [23 Jul, 2010]
is a bundle of icons, designed with usage-under-Calibre in mind, allowing users to visually refine the full-fledged utility. The Sphere icons present the user a rounded icon style with multiple used colors and a care for details, while retaining usability (being easily identifiable as their intended purpose) and a pleasant-to-the-eye look & feel.
Gnome-friendly icon set
. This is the partial set that inspired Kovid to add the user customization of icons feature.
[How to] Make e-book app ‘Calibre’ look nicer in Ubuntu
Originally Posted by AmishSexy
Calibre can look a bit out of place on a gnome desktop due to the oxygen icons that come bundled with it. I've put together a partial replacement icon theme based on Tango and Humanity. Enjoy!
. This easy on the eyes icon set is a drop-in replacement for the calibre icons.
Metro Skin icon set
. The actual icon set is attached to this post
Originally Posted by Steppa
I am big fan of the clean monochromatic look of Metro UI (used in Windows Phone 7). I have skinned about 80% of Calibre on the same principle and here are the results.