Right now, Boost Union implements a mix of forcing child themes to reimplement some settings and, in case of other settings, explicitly takes Boost Union's settings.

The first case could lead to unnecessary duplication of settings code.
The second case could maybe lead to duplication of function code if one would prefer to implement a theme's settings as "all settings for theme appearance should be configurable in a theme's settings pages).

I propose a function that returns a child theme's setting value if that setting exists in that child theme and otherwise returns Boost Union's setting value.

I'll prepare a pull request with that function (probably in lib.php or locallib.php, even if I like autoloaded classes wrapping this stuff, i.e. "class settingsutils"), and if the idea is accepted, I'd complete the PR with a bigger commit replacing all settings use with using this function.