Attempts at enabling Bricks Builder for Popup Maker, needed safe place to store the code outside Popup Maker core files for now.

functions.php

<?php
/**
 * Check if the current page is the Bricks editor preview.
 *
 * @return bool
 */
function pm_is_bricks_editor_preview() {
		// phpcs:ignore WordPress.Security.NonceVerification.Recommended
	return isset( $_GET['bricks_preview'] );
}

/**
 * Get the current post ID.
 *
 * @return int|null
 */
function pm_bricks_get_current_post_id() {
	global $post;

	$post_id = isset( $post ) ? $post->ID : null;

	if ( ( bricks_is_builder_iframe() || pm_is_bricks_editor_preview() ) && class_exists( '\Bricks\Helpers' ) ) {
		$preview_post_id = \Bricks\Helpers::get_template_setting( 'templatePreviewPostId', $post_id );

		if ( $preview_post_id ) {
			$post_id = $preview_post_id;
		}
	}

	return $post_id;
}

function pm_bricks_should_output_custom_vars() {
	if ( ! function_exists( 'get_field' ) ) {
		return false;
	}

	$checks = [
		'is_singular'       => is_singular(),
		'bricks_is_builder' => bricks_is_builder(),
		'bricks_previews'   => bricks_is_builder_iframe() || pm_is_bricks_editor_preview(),
	];

	return in_array( true, $checks, true );
}

/**
 * Retrieve custom header variables based on ACF fields.
 *
 * @return array<string,string>
 */
function pm_get_template_header_variables() {
	if ( ! function_exists( 'get_field' ) ) {
		return [];
	}

	// Ensure we're on a single post or page
	$post_id = pm_bricks_get_current_post_id();

	if ( ! $post_id ) {
		return [];
	}

	// Retrieve the color scheme field
	$color_scheme = get_field( 'header_color_scheme', $post_id );

	$variables = [];

	// Proceed only if the color scheme is set to 'custom'
	// Adjust the condition if your 'custom' value differs
	if ( '' === $color_scheme ) {
		// Retrieve individual color fields
		$header_background_color  = get_field( 'header_background_color', $post_id );
		$header_text_color        = get_field( 'header_text_color', $post_id );
		$header_accent_color      = get_field( 'header_accent_color', $post_id );
		$header_accent_text_color = get_field( 'header_accent_text_color', $post_id );

		$variables = [
			// Foreach if its falsy, set to null.
			'header-background-color'  => $header_background_color ? $header_background_color : null,
			'header-text-color'        => $header_text_color ? $header_text_color : null,
			'header-accent-color'      => $header_accent_color ? $header_accent_color : null,
			'header-accent-text-color' => $header_accent_text_color ? $header_accent_text_color : null,
		];
	}

	return array_filter( $variables );
}


/**
 * Outputs custom header styles based on ACF fields.
 *
 * @return void
 */
function pm_render_custom_css_variables() {
	if ( is_admin() ) {
		return;
	}

	$overrides = pm_get_template_header_variables();

	// Check if at least one custom color field is set
	if ( ! empty( $overrides ) ) {
		// Start building the CSS output
		$custom_css = "<style id=\"custom-header-stylings\" type=\"text/css\">\n:root {\n";

		$custom_css .= "/* Custom Stylings Based on ACF Template Fields */ \n";

		// Loop through each variable and append it
		foreach ( $overrides as $name => $value ) {
			// Set the override variable
			$safe_value  = esc_attr( $value );
			$custom_css .= "    --$name-override: {$safe_value};\n";
		}

		$custom_css .= "}\n</style>\n";

		// Output the custom CSS
		echo $custom_css;
	}
}

/**
 * Add custom global variables to the Bricks global variables option.
 *
 * @param array{
 *  id: string,
 *  name: string,
 *  value: string,
 * }[] $value The global variables.
 *
 * @return array{
 *  id: string,
 *  name: string,
 *  value: string,
 * }[]
 */
function pm_bricks_filter_global_variables_option( $value ) {
	if ( ! $value ) {
		return $value;
	}

	if ( ! is_array( $value ) ) {
		$value = [];
	}

	if ( ! did_action( 'wp' ) ) {
		return $value;
	}

	$overrides = pm_get_template_header_variables();

	if ( ! empty( $overrides ) ) {
		foreach ( $value as $key => $val ) {
			$name = $val['name'];

			if ( array_key_exists( $name, $overrides ) ) {
				$value[ $key ]['value'] = $overrides[ $name ];
			}
		}
	}

	return $value;
}

/**
 * Modify the global variables used in the Bricks CSS.
 *
 * @return void
 */
function pm_modify_bricks_css_global_variables() {
	if ( class_exists( '\Bricks\Database' ) ) {
		$overrides = pm_get_template_header_variables();

		if ( ! empty( $overrides ) ) {
			/**
			 * Global variables.
			 *
			 * @var array{
			 *  id: string,
			 *  name: string,
			 *  value: string,
			 * }[] $globalVariables
			 */
			$global_variables = \Bricks\Database::$global_data['globalVariables'];

			foreach ( $global_variables as $key => $val ) {
				$name = $val['name'];

				if ( array_key_exists( $name, $overrides ) ) {
					$global_variables[ $key ]['value'] = $overrides[ $name ];
				}
			}

			\Bricks\Database::$global_data['globalVariables'] = $global_variables;
		}
	}
}

function pm_merge_global_variables( $global_variables ) {
	$overrides = pm_get_template_header_variables();

	do_action( 'qm/debug', 'GlobalVariables' );
	do_action( 'qm/debug', $overrides );
	do_action( 'qm/debug', $global_variables );

	if ( ! empty( $overrides ) ) {
		foreach ( $global_variables as $key => $val ) {
			$name = $val['name'];

			if ( array_key_exists( $name, $overrides ) ) {
				$global_variables[ $key ]['value'] = $overrides[ $name ];
			}
		}
	}

	return $global_variables;
}

// add_filter('bricks/load_global_variables', 'pm_merge_global_variables');
// add_filter('bricks/get_global_variables', 'pm_merge_global_variables');

// add_filter(
// 'bricks/api/get_templates_data', function ( $templates_data ) {
// $overrides = pm_get_template_header_variables();

// if (! empty($overrides) && is_array($templates_data['globalVariables']) ) {
// foreach ( $templates_data['globalVariables'] as $key => $val ) {
// $name = $val['name'];

// if (array_key_exists($name, $overrides) ) {
// $templates_data['globalVariables'][ $key ]['value'] = $overrides[ $name ];
// }
// }
// }

// return $templates_data;
// }
// );

add_action( 'wp', 'pm_modify_bricks_css_global_variables', 0 );
add_action( 'wp_head', 'pm_render_custom_css_variables', 2 );
add_filter( 'option_bricks_global_variables', 'pm_bricks_filter_global_variables_option' );