Add the following code snippet in your theme style.css

/* bbpress specific styling */

#bbpress-forums,
#bbpress-forums ul.bbp-forums,
#bbpress-forums ul.bbp-lead-topic,
#bbpress-forums ul.bbp-replies,
#bbpress-forums ul.bbp-search-results,
#bbpress-forums ul.bbp-topics,
div.bbp-template-notice li,
div.bbp-template-notice p,
div.bbp-breadcrumb,
div.bbp-topic-tags {
	font-size: .9rem;
	line-height: 1.5;
}

#bbpress-forums .bbp-forum-info .bbp-forum-content,
#bbpress-forums p.bbp-topic-meta,
span.bbp-admin-links a {
	font-size: .9rem;
}