It seems like a new feature was added to the PMSE site, but I can not found it's meaning. Some questions are surrounded by a light blue box. What does it mean?
This is highlighting questions with tags that are in your favorite tags. If you haven't manually added any favorite tags, the system will detect when you have a certain level of activity in a tag and infer one for you. If you manually add a favorite tag, the system will not infer a tag for you.
You can customize your favorite (and ignored tags) in your profile. You can also customize these settings in the right sidebar of the homepage and the /questions listing.
So, if you would prefer not to have any questions highlighted, you can simply add a nonsense "tag" to your favorites. Perhaps unicorn? (: