I've installed 4.3.10rc1 and have styled that using ploneCustom.css in the ZMI, the way I was used to working prior to Plone 5. I will have another look at Dexterity theming when I have the time.
To answer some questions, I copied Barceloneta theme in the Theming control panel in site setup. I built or compiled a css file called styles.css from the less file. Everything seemed to be in order, and when I previewed the theme it previewed correctly and as expected. When I enabled the theme I got an un-themed Plone. This was after much reading and research. It is just easier for me to use 4.3.10 and get on with the project. I do thank everyone for their help and appreciate their time and suggestions.