PR for multiple flavour and child-theme related issues

[abias]

This PR fixes multiple flavour and child-theme related issues:

* 2024-12-31 - Child theme support: Fully replicate Boost Union's extra SCSS if a Boost Union Child theme is the current theme, resolves #718, resolves theme_boost_union_child/#5.
* 2024-12-31 - Bugfix / Child theme support: The theme_boost_union_get_pre_scss() and theme_boost_union_get_extra_scss() function used $theme->settings although they should not do that anymore, resolves #791
* 2024-12-31 - Tests: Increase the test coverage for the background image setting, helps to resolve theme_boost_union_child/#5
* 2024-12-31 - Bugfix: In flavours, not setting the brand color / bootstrap colors in a flavour did not result in the global brand color / bootstrap colors being served properly, resolves #790.
* 2024-12-30 - Improvement: Add more flavour settings (Activity icon colors, navbar color, background image position), resolves #789.