Access css residing in the jar as vaadin terminal Resource

I’m trying to access css in vaadin application.
The css is bundled in custom-theme.jar that exists in WEB-INF/lib of portlet
When jar is expoded, the path looks like \classes\VAADIN\themes\custom-theme\styles.css

How can I access this css residing in the jar as com.vaadin.terminal.Resource ?

The application is jsr-286 vaadin portlet running on Liferay 6.0.6 tomcat bundle.
Vaadin version used is 6.7.0

Already tried following to no avail.

ThemeResource customStyles = new ThemeResource("../VAADIN/themes/custom-theme/styles.css");
String path = "../VAADIN/themes/custom-theme/styles.css";
Resource style = new ClassResource(path , this);
String path = "../VAADIN/themes/custom-theme/styles.css";
ClassLoader classLoader = Thread.currentThread().getContextClassLoader(); 
InputStream input = classLoader.getResourceAsStream(path);

Thank you.

Actual need was for cssinject addon usage.
Slightly modified the third option and it worked.
So I could actually do away with the requirement of accessing the css as vaadin terminal resource.
The custom theme including images are bundled in jar.

// you should know how your custom-theme jar contents are packaged to arrive at this path
String path = "/VAADIN/themes/custom-theme/styles.css"; 
InputStream cssInputSream = this.class.getResourceAsStream(path);
String css = IOUtils.toString(cssInputSream);
//and then
new CSSInject().setValue(css);