I am an experienced Linux admin, but a complete WordPress noob.
I had a working wordpress installation. (perhaps a bit kludged with symlinks in places because I don't know how to set things up). The customer requested an additional site on the same server. So I figured out how to run the script (/usr/share/doc/wordpress/examples/setup-mysql) to add another blog/site.
This appeared to work. However, when the customer went to add a new theme to the site, they found that they couldn't do so. They got an error about the themes directory not being found.
Much poking around later, I discovered that /usr/share/wordpress/wp-content was owned by 'root:www-data' and (I think) everything under it chmod'ed 770. (I don't know if this is normal and good).
Some experimentation showed that when using FTP (from the WordPress 'Manage Themes' page) to upload the new theme, the files were attempting to be written as the FTP upload user. I added that user to the www-data group, and now the themes can be uploaded and are unpacked just fine.
Problem is, any of the new themes which have been uploaded since the new blog/site was added, render as a white screen (with the exception of the Weaver II theme which renders a beige screen). The old themes, installed before I added this new blog, work fine.
Even if I delete an old theme, then re-download-and-install it, it works fine. No new themes work tho.
I'm guessing this must be comething to do with the database; but I am completely clueless about that.
What may I try next to troubleshoot this? I don't know if anything else is broken other than themes; that's all which has been brought to my attention at the moment.
I am running WordPress 2.9.2 on Ubuntu Lucid.
I have no plugins running, and have made no modifications by hand to any of the WordPress code.