opaque background, multiple component updates

src/plainui/jinja2/plainui/components/list_assemblies.html

src/plainui/jinja2/plainui/components/list_events.html

src/plainui/jinja2/plainui/components/list_rooms.html

src/plainui/jinja2/plainui/components/listbox.html

src/plainui/jinja2/plainui/components/markdown.html

src/plainui/jinja2/plainui/components/tile_board.html

src/plainui/styles/_util-classes.scss

src/plainui/styles/components/_event-info.scss

src/plainui/styles/utils/_globals.scss