Problem / Motivation
The Appearance admin page now presents a long list of available themes. Uninstalled themes group in particular is long and many "Test themes" doesn't have screenshots.
This page is therefore hard to read and one have to scroll quite a bit in order to see the ADMINISTRATION THEME form at the bottom of the page.
Proposed solution
I propose to make theme_groups "collapsible" in system-theme-page.html.twig.
Installed themes group should be open by default whereas Uninstalled themes group should be closed by default.
