My simple needs were well served by the old Ultimate Maintenance Mode plug-in. I was not happy at the news that it would be deprecated and I would have to use a new plug-in in with more bells and whistles. I started looking at other Maintenance Mode plug-ins.
I can report that I could find none that would do what I want as easily as using the SeedProd Coming Soon Page & Maintenance Mode plug-in. I made changes and found settings easier and I don't have to go into bootstrap.min.css any more to lighten my backbround page. :-)