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.
The file name should have an underscore prefix: _email-inline.less
After writing your styles you should remove static files
rm -rf var/view_preprocessed/* pub/static/*
Please add email-inline.css instead of email-inline.less on
app/design/MyVendor/MyTheme/web/css
Please try to add important on your custom css changes because if Magento default CSS priority before your CSS in that case you changes not reflected so please check with important.
Here is example for you reference.
background-color: red !important;
Hope it will work for you!
Please add your css in _email.less file. Below is the file path.
app/design/frontend/{themename}/{namespace}/web/css/source/_email.less
If _email.less is not exists on this file then copy it from the parent theme.
