On theme installation, a dialog for handling the plugin installation should appear. It will lead you to the plugin page and give you an option to enable it after you install it and restart Steam. In case the plugin will stop working, it will give you the option to revert your theme to default.
This theme requires a plugin for the top bar and some other additional elements. The menu bar, links to a game's store page, community hub, etc. and some other things will not be accessible if not installed. Learn to read if you missed this.
Based on a mockup by MondySpartan: https://www.deviantart.com/mondyspartan/art/Steam-Office-2007-Styled-Mockup-1060261068
Other previews
Use the Change Window Params plugin and enable "Use system titlebar".