Even most of the themes use BootstrapBase not all have the same structure and, not all have settings to manage the site from backend.
So as the forum rules show, first you must indicate theme used and version, and the moodle version, to help you.
On the other hand, when I replied you about the background is hardcoded in the theme (which is still pending of change) it doesn't mean you can't change the colour. You ALWAYS can do it using the custom CSS box available in most of the Moodle Themes. Adaptable only includes settings for the tiles view, not for the default moodle view (which is part of the theme itself)
Also, using Firebug, you can change many theme styles. And if you want to create a 'dark' theme then you will need to make a lot of changes and most of them are not in the theme settings. You could achieve only using custom CSS.
And yes, if you change later the theme will need to rewrite all the styles again. So the theme used is really important.