Hello,
First of all, i'm glad you tried many solutions, and thanks for your interest in our platform.
You're doing it right, it can be a small thing: when looking at your description, there are several factors that can disrupt the smooth running of your task.
By probability order:
1/ Your jar is overrided by another one (ours).
Your way to require a bundle is not used anymore. You don't need to add it in the Manifest.MF anymore. Once your theme-contrib.xml file is called in the manifest, be sure your component name is unique, and add a require tag on ours. You can see that on the 2nd and 3rd lines from this theme-contrib.xml file example: http://doc.nuxeo.com/display/KB/Adding+your+own+CSS#AddingyourownCSS-Declarethefile
2/ Your component name is not unique.
Check the server.log, it will be noticed that there is a duplicate, you will see which jar is picked by the server.
Same solution than previously, be sure your component name is unique, and add a require with the name of the component you want to override.
3/ The tool you use to unzip and zip your .jar file doesn't do the job correctly.
We made the test (on linux and mac OS), unzip, make a change and zip a .jar, restart the server, it's working well. It can be a Windows tool issue, i don't know what kind of tool you're using.
If none of these solutions resolve your issue, try to describe your environment, check if you see something related in the log even if the server starts and tell us.
Thanks
