The theme installation tools in this update have been replaced by links to the best places to get themes for the Original Software, 240p Rockbox and 360p Rockbox themes. These are the themes.innioasis.app browser-based theme downloader for the original software, the Rockbox.org themes page for 240p iPod themes and the Rockbox-y1 Themes repository.
You'll find helpful buttons / shortcuts for these in Toolkit which can be found in the start menu and Updater app itself.