It is not possible the change the canvas background color at the moment.
But I would recommend the use of layers. These are colored areas that are used to group schema objects together that belong together. If you make regular use of these layers, you will hardly see the neutral white background.
Please see here for an example to see what I mean.
http://dev.mysql.com/workbench/wp-content/uploads/wb_diagam_fk_highlight1.png
It not only helps you to keep your models more organized, it also helps to quicker navigate on the canvas because the eagle-eye view becomes more meaningful.
Hope this helps.
If you still need to change the background color, please file a feature request on
http://bugs.mysql.com
Michael Zinner, Team Lead, Developer Tools
MySQL AB, www.mysql.com
Are you MySQL certified? www.mysql.com/certification
