The solution <stylesheet src="CustomStylesheet.css" />
is deprecated and did not work with the new superdevmode.
A solution that worked for me (using GWT 2.7) was to create a new custom theme:
projectPackage/themes/MyCustomTheme/MyCustomTheme.gwt.xml
<module>
<stylesheet src="gwt/MyCustomTheme/MyCss.css"/>
</module>
projectPackage/themes/MyCustomTheme/MyCustomThemeRessources.gwt.xml
</module>
projectPackage/themes/MyCustomTheme/public/gwt/MyCustomTheme/MyCss.css
(Note: removing the gwt/MyCustomTheme/ part of the path worked in devmode but didn't work in deployed version, of cause you can still rename 'MyCustomTheme' to something of your liking)
The css file you want to use
Project.gwt.xml
<!DOCTYPE module PUBLIC "-//Google Inc.//DTD Google Web Toolkit 2.0//EN"
"http://google-web-toolkit.googlecode.com/svn/releases/2.0/distro-source/core/src/gwt-module.dtd">
<module rename-to="Project">
(...)
<!-- Inherit GWT theme. e.g. the Clean theme -->
<inherits name='com.google.gwt.user.theme.clean.Clean'/>
<!-- Our custom theme -->
<inherits name='projectPackage.themes.MyCustomTheme.MyCustomTheme'/>
(...)
</module>
Note: You can get a sample custom theme using http://gwt-theme-generator.appspot.com/ and extracting the downloaded .jar file.