@Bruno,

What I would recommend is moving the theme directory to another directory name in your local filesystem. For example if you have a module directory called website_multi_image that you're working on you could do the following:

sudo mv /opt/odoo/custom/addons/website_multi_image /opt/odoo/custom/addons/website_multi_image_BACKUP

you should then be able to restart your odoo-server from the command line (or openerp-server depending on your setup) and access your backend again. This is a workaround i've used countless times when developing odoo theme modules.

If the issue is really persistent then you could try reverting or fixing the changes you made that threw the error in the local filesystem and then upgrading the module only at the command line. This involves starting the server with the following structure (eg. using website_multi_image) for example:

./openerp-server --update=website_multi_image -c /etc/odoo-server.conf

Please comment back if you run into any issues with the above methods. I'd also recommend getting on the pre-order for this book:

https://www.packtpub.com/big-data-and-business-intelligence/odoo-essentials

I believe this will be targeting developers and it looks like it will be a great resource for theme development.