I assume you know that that style sheet is for the admin screens only?
If not, the ones you want to look at are:
templates/<template directory>/styles/style.css and
<template directory> is most likely "default", but can be changed in the config properties.
<language.css> is the language specific css, e.g. en_US.css that defines the button images with words on them.
Hint: View source is your friend in determining this.
Also, many of the template htm files call images directly instead of using css. For example the gray-blue line at the bottom of the screen is an image. You many need to change the images or remove them from the template files. [originally posted on jforum.net by monroe]