Hi Peter,
I'm sure the background of the frontpage has not changed color in Adaptable, since it was made a few years ago, as the background colour is actually coded in the Bootstrapbase theme. And since you are using Moodle 3.3 then the colour will still be white as it always has been so.
The only problem you will have is that the colour is set in the HTML, body and so everthing page has a white background.
You could try the following CSS:
body#page-site-index { background-color: blue !important;}
Hope this helps?
Cheers
Mary