I was typing a document-- suddenly the task bar with bold,underline etc disappeared. A huge, intrusive New Theme bar with pictures is now at the top of every new page.
It’s simply the Image Gallery 
To hide it click on the menu Tools > Gallery (7th item)
Choose Tools - Gallery or on Standard Bar, click the Gallery icon
See also LibreOffice Help: Showing and Hiding Docked Windows
Click the button on the edge of the docked window to show or hide the docked window…
This looks to me like a malware. Did you run a virus check on your machine?
Which OS and LibO version are you using?
