Based on a mockup by MondySpartan: https://www.deviantart.com/mondyspartan/art/Steam-Office-2007-Styled-Mockup-1060261068
This theme (optionally) 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.
It's going to be available in a separate plugin in the future. It's not released yet because it doesn't start early enough to modify window.open before the main window appears (but works for everything else). The screenshot only includes it so I don't have to take another one.
The theme features (almost) 1:1 context menus, tooltips & scrollbars. Other controls coming soon in a future version.