My iMac has started covering text in Word with an opaque grey rectangle when I drag my cursor over text or double/triple click it to make some sort of change instead of the see-thru blue highlighting it has always done. How do I get the see-thru blue back

First, restart the Mac. If that doesn't resolve it confirm that both Mac Office & your edition of macOS are fully updated. If you include their Version Numbers in a reply here they can be confirmed.

The system is seriously behind on updates. Version 15.7.2 is the current release of Sequoia. There never was a 16,1 of Mac Office but the present build is 16.103.4 or later depending on Update Channel.

I believe bringing the Mac to a current status will resolve the issue.

Thanks, Bob. I have been using help>check for updates in the Word menu bar. This time I downloaded the AutoUpdate 4.81.2 link you provided and tried to update again. Before this update I was using Word version 16.100.4 and after, I've got Word version 16.104 but the update ended with the following Update Error..."Some apps have encountered errors" after about 90 minutes. And I'm sorry to say the problem with the opaque grey box persists in Word.

Thanks, Bob, for your answer. I have been using Word is 16.100.4. Tried to update but after 3 hours got error message saying can't update. Guessing next step is deleting Word and redownloading, but can't get to a human at Microsoft to guide me through the process.

Click the Apple icon at the top left.

Select System Settings - Appearance.

In there, you can change the Text Highlight color on a Mac computer.