I have been using libreoffice for some years now with no complaints whatsoever. However I recently upgraded to 5.0 from 4.4 and the UI is just horrific!! is it just my version (debian) it seems like I was thrown back to office 2000 or win98. What has happened?
go to Tools menu >> Options >> in Sidebar you find (Libreoffice >> under it View)
Check if you can find a better theme under icons and style
Check Q64348 to see if you have been hit by the GTK3 issue.