created an attachment (id=163425)
project that shows the problem.

this is a bit of a tricky one to re-create, but it is a very common case.  typically when doing development, the resource_home is set to the workspace_loc.  

when you do this, and you use a library that has theme built from css files, then there is an error.  i have re-created the condition in the attached zip file to demonstrate.  if you unzip the project into your workspace and then open the report library you should see the problem with the style.  

notice: this project has enabled project specific settings for the workspace home.  the same will occur if you use workspace configuration settings using either a hard coded path or variables.

to duplicate:
1) create a new birt project
2) override the project settings for reports to point to the workspace_home and not the project directory
3) create a small css file
4) create a new report library
5) create a new theme, use the css in that theme
6) add a label to either the header or footer of the master page and then reference one of the styles in the css (should work).
7) close the library file
8) re-open the library.  examine the theme and you should see a red x indicating that the css file can not be found. 
9) examine the label controls, similar problem they can't find the style

to resolve you have to go into the xml, remove the css file, and then re-add the file.  that will work until you open and close the file.
this is fixed by fix for bug 291558.
verified in 2.6.0.v20100527-0630
no error now.