Email template styles are not working

I’ve not changed the core header.html which is in the vendor directory.

According to the Magento 2 dev docs, I must create “email-inline.less” in app/design/MyVendor/MyTheme/web/css directory, however, the styles are not being applied to the sent email.

I’m wondering if email-inline.less file will be compiled automatically or I have to compile it first to the css file? I’ve actually made seperate email-inline.css file, but styles were not working with this one too.