Fix dark mode for GA import

lib/plausible_web.ex

...le_web/templates/site/import_from_google_confirm.html.eex

...mplates/site/import_from_google_user_metric_form.html.eex

...b/templates/site/import_from_google_view_id_form.html.eex

lib/plausible_web/templates/site/settings_general.html.eex

lib/plausible_web/views/form_helpers.ex