Convert all urls in layout css to base64 encoded data urls.

functions.php

<?php // this lines just here so the formatting works, ignore it.

add_filter( 'fl_builder_render_css', function( $css, $nodes, $global_settings, $include_global ) {

	if ( isset( $_GET['fl_builder'] ) ) {
		return $css;
	}
	// keep original in case this goes tits up
	$original = $css;
	$parse    = preg_match_all( '/(https?:\/\/.*\.(?:png|jpe?g|gif|webp))/', $css, $urls, PREG_PATTERN_ORDER );

	if ( ! $parse ) {
		return $original;
	}

	foreach ( $urls[0] as $url ) {
		$path = str_ireplace( home_url(), untrailingslashit( ABSPATH ), $url );
		if ( is_file( $path ) ) {
			$type       = exif_imagetype( $path );
			$typestring = null;
			switch ( $type ) {
				case IMG_GIF:
						$typestring = 'image/gif';
					break;
				case IMG_JPG:
						$typestring = 'image/jpg';
					break;
				case IMG_JPEG:
						$typestring = 'image/jpeg';
					break;
				case IMG_PNG:
						$typestring = 'image/png';
					break;
				case IMAGETYPE_WEBP:
						$typestring = 'image/webp';
						break;
				default:
						$typestring = 'unknown';
			}
			if ( 'unknown' !== $typestring ) {
				$data = sprintf( 'data:%s;base64,%s', $typestring, base64_encode( file_get_contents( $path ) ) );
				$css  = str_replace( $url, $data, $css );
			}
		}
	}
	return $css;
}, 10, 4 );