Can you provide the site url you are having this issue with?
I updated my site not long ago, and didn't have any visual changes that took effect. This could be a mod interfering with the style.
I have done a lot of forum projects, and as a courtesy, I always give the option of installing a new mod that phpbb put out, A mod that allows you to add/remove mods without having to change the 1st line of code, just upload it, go into the ACP, and install it. this helps with problems like these.