You can follow these step
vendor/magento/theme-frontend-blank/web/css/email.less
vendor/magento/theme-frontend-blank/web/css/email-inline.less
into:
app/design/frontend/YOUR_VENDOR/YOUR_THEME/web/css/
and removed this line from both files '@import 'source/_theme.less'.
Cause by doing this we are separating theme css and email css.
Last run the following commands:
php bin/magento maintenance:enable
rm -rf var/cache/* var/page_cache/* var/di/* var/generation/* var/view_preprocessed/* generated/* pub/static/*
php -d memory_limit=12G bin/magento setup:upgrade
php -d memory_limit=12G bin/magento setup:di:compile
php -d memory_limit=12G bin/magento setup:static-content:deploy -f
php bin/magento c:f
php bin/magento c:c
chmod -R 0777 var/ generated/ pub/static
php bin/magento maintenance:disable
