Customize Email Templates
UM email templates by default not support <html>, <head>, <body> tags. You can customize via wp-admin screen only <body> inner HTML.
If you have big custom email template with your "<html>, <head>, <body>" tags and custom attributes you may use UM custom filters for make this works properly.
1) Via 'um_email_template_body_attrs' filter you may edit your email template body attributes:
2) Via 'um_email_template_html_formatting' filter you may edit your email template header: