Issue #2421373 by mortendk, Wim Leers: rename *.icons.css files to *.icons.theme.css

core/modules/contextual/contextual.libraries.yml

core/modules/contextual/css/contextual.icons.theme.css

core/modules/quickedit/css/quickedit.icons.theme.css

core/modules/quickedit/quickedit.libraries.yml

core/modules/shortcut/css/shortcut.icons.theme.css

core/modules/shortcut/shortcut.libraries.yml

core/modules/toolbar/css/toolbar.icons.theme.css

core/modules/toolbar/toolbar.libraries.yml

core/modules/user/css/user.icons.theme.css

core/modules/user/user.libraries.yml