The theme plugin that inherit boost theme doesn't get booststrap style like boost theme.
Since we have a lot of themes that inherit from the Boost theme.
It's impractical to override the format_topcoll method(as Gareth suggest in the comment in issue CONTRIB-6954) in every single themes.

Reproduce step:

1. Install theme that inherit from boost theme.
2. Use the theme from step 1.
3. Create a course with "Collapsed Topic" course format.
4. Go to the course.
5. Compare difference between two theme in course collapse sections by switch theme between Boost and child theme. 
6. Inspect <ul class="ctopics topics row"> element in browser.
7. List element in Boosts theme classes like <li class="section main clearfix col-sm-12 col-md-12 col-lg-12">
8. List element in child theme have the classes like <li class=""section main clearfix span12">

renderer.php: 81

if (strcmp($page->theme->name, 'boost') === 0) {

    $this->bsnewgrid = true;

}

The use of format_topcoll_renderer::$bsnewgrid 

protected function get_column_class($columns) {

    if ($this->bsnewgrid) {

        $colclasses = array(

            1 => 'col-sm-12 col-md-12 col-lg-12',

            2 => 'col-sm-6 col-md-6 col-lg-6',

            3 => 'col-md-4 col-lg-4',

            4 => 'col-lg-3');

    } else {

        $colclasses = array(1 => 'span12', 2 => 'span6', 3 => 'span4', 4 => 'span3');

    }

    return $colclasses[$columns];

}

Suggest fix at renderer.php: 81

if (strcmp($page->theme->name, 'boost') === 0  || (isset($page->theme->parents) && in_array('boost', $page->theme->parents))) {

    $this->bsnewgrid = true;

}