Custom html not retaining css in <head> section needed for mobile-friendly emails

Custom html not retaining css in <head> section needed for mobile-friendly emails

Our emails use css in the <head> section to customize the email to display nicely on mobile devices (using different styles for different viewports). The emails work perfectly outside of ZC, sent from Mailchimp or any other mainstream email provider.

When I paste the custom html into the html box in Zoho Campaigns, ZC removes the <head> section, which removes the mobile-specific styles, making the emails not render properly on mobile. It appears that when I import the same html file, ZC retains the <head> section, so the email renders nicely on mobile, but then it is no longer editable in Zoho, which means that I can no longer use ZC's html editor.

How do I create a custom html email template in ZC that is editable and that retains the <head> section?

Seems like this is a pretty basic requirement for any email editor.

Thanks for your help.