[REF] *: use 'light' color and reset 'gray' palette

addons/mail/static/src/scss/chatter.scss

addons/mail/static/src/scss/composer.scss

addons/mail/static/src/scss/discuss.scss

addons/mail/static/src/scss/mail_activity.scss

addons/mail/static/src/scss/thread.scss

addons/web/static/src/scss/bootstrap_overridden.scss

addons/web/static/src/scss/kanban_examples_dialog.scss

addons/web/static/src/scss/kanban_view.scss

addons/web/static/src/scss/primary_variables.scss

addons/web/static/src/scss/secondary_variables.scss