The Beaver Builder Themes default values in the Customizer can be changed with the fl_theme_add_panel_data filter. These are the values when you click on the default button that return a value set from the original code.

The filter is found bb-theme/classes/class-fl-customizer.php and takes 2 parameters $key and $data, the former being the customizer panel and the latter the actual customizer settings.
The $key(s) are …
- fl-general
- fl-header
- fl-content
- fl-footer
- fl-code
So the filter is expressed like so…
add_filter('fl_theme_add_panel_data','bt_customizer_options_filtered', 10, 2 ); 
/**
 *  Filtering the Customizer
 *  Not working for theme if inside after_setup_theme function
 *  Changing the defaults on the live preset.
 *  @since 2.0.0
 */
function bt_customizer_options_filtered( $data, $key ) {
    if ( 'fl-general' == $key ) {
        // Do something with the $data.
        // Layout
        $data['sections']['fl-layout']['options']['fl-content-width']['setting']['default']='1200';
        $data['sections']['fl-layout']['options']['fl-scroll-to-top']['setting']['default']='enable';
        $data['sections']['fl-layout']['options']['fl-awesome']['setting']['default']='fa5';
        $data['sections']['fl-layout']['options']['fl-framework']['setting']['default']='bootstrap-4';
        // Background
        $data['sections']['fl-body-bg']['options']['fl-body-bg-color']['setting']['default']='#ffffff';
        // Accent Color
        $data['sections']['fl-accent-color']['options']['fl-accent']['setting']['default']='#EB5D50';
        $data['sections']['fl-accent-color']['options']['fl-accent-hover']['setting']['default']='#2C848D';
        // Fonts
        $data['sections']['fl-heading-font']['options']['fl-title-font-family']['setting']['default']='system-ui';
        $data['sections']['fl-heading-font']['options']['fl-heading-font-family']['setting']['default']='system-ui';
        $data['sections']['fl-body-font']['options']['fl-body-text-color']['setting']['default']='#333333';
        $data['sections']['fl-body-font']['options']['fl-body-font-family']['setting']['default']='system-ui';
        $data['sections']['fl-body-font']['options']['fl-body-font-size']['setting']['default']='18';
    if ( 'fl-header' == $key ) {
        $data['sections']['fl-header-logo']['options']['fl-logo-font-family']['setting']['default']='system-ui';
        $data['sections']['fl-nav-style']['options']['fl-nav-font-family']['setting']['default']='system-ui';
        $data['sections']['fl-nav-style']['options']['fl-nav-hover-color']['setting']['default']='#999999';
    }
 
    return $data;  
 }
}
The nested arrays are cascaded until the default setting which is then assigned the new default value.
Above both the fl-general and fl-header are targetted with if statements and various default values set.
You can view the structure of various nested arrays in the normal array presentation in the bb-theme/includes directory such as bb/theme/includes/customizer-panel-general.php file.









