I trust you created a backup of the project before upgrading. You may need that.
Try deleting the CPD file and then reopening the project.
If that does not improve things, try Tools > Update DHTML.
In the Topic List you can customise the columns shown. Select CSS there and make sure they are all correct.
In one topic that is wrong select another CSS and save. Then revert to the required CSS. Does that one then play nicely.
Where is your project store? Is the path very long?
See www.grainge.org for RoboHelp and Authoring tips
How are backgrounds defined in your CSS? And on which elements have you set backgrounds?
Do your topics contain any inline styling overwriting the CSS? (Check for style attributes in HTML and a style block in the top of your file.)
After EXTENSIVE research and trial and error, the Help Desk person and myself found that when the project is compiled, the line:
the “text/css” was removed and not present in the final page source code HTML:
We replaced the line from another file, and deleted this line and recompiled, and viewed the files and the background displays properly.
We then copied the very same line and copied it over the original line and recompiled, and viewed the files and the background displays properly. There is apparently a glitch in the system that doesn’t agree with our project.