Windows Live Writer makes a local copy of your website images and style sheets everytime you click on the Update Theme button located in the Blog Account ribbon.
You can find your local CSS file(s) in the following folder on Windows 7:
C:UsersYour NameAppDataRoamingWindows Live Writerblogtemplates
If you only have one blog, you are lucky. Get into the only folder you find there. If you have many blogs it will be a trial and error process to find out the right folder where your CSS files are located.