I presume your css file is called base.css?
Personally, I would recommend adding a parent theme (probably base)
$THEME->parents = array('base');
because at the moment, with no parent theme, you have none of the layout files declared other than the base one, and if as you say your css file is empty, then you have no css to fall back to either. If you declare a parent theme, then your theme can fall back to the layout files used in the parent, but without any parent, I don't think it will work as there are no layout files declared anywhere (@Mary or @Gareth - would Moodle fall back to the base layout for other page layouts if none are declared ie there is no parent, even base/bootstrapbase? I don't think it would, but haven't specifically tested that.
EDIT:
just tried in a Moodle3 site by removing the layout files from bootstrapbase config.php and selecting that as the theme - error messages, but the page is there so must be picking up the base layout, I guess). Hmmm, OK.
As well as the Main Content token changing that Mary has already mentioned, from somewhere around Moodle 2.2 or 2.3 a version.php became a required file in the setup of your theme and your theme will not install without it.
If you are using such an old version of Moodle as 2.0 or 2.1, please see https://docs.moodle.org/dev/Creating_a_theme for instructions
If you are using a newer version (bearing in mind that Moodle is now at v.3 and even 2.7LTS is only security fixes now), then please see https://docs.moodle.org/dev/Clean_theme, or the readme file in the Clean theme itself.
TBH if you are using something as old as 2.0 or 2.1 then I would ask the question 'why?' If the purpose of your theme is for testing and studying, surely you should be looking at an upto date version of Moodle and how themes work in that.
Richard