How to create kmail header based on Grantlee:
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You can use directly headerthemeeditor

When you want to add css file or js file you need to use:
"<link href="{{ header.absoluteThemePath }}style.css" rel="stylesheet" type="text/css" />"
header.abasoluteThemePath defines path where is installed themes. KMail doesn't load relative path


We have some variable which can use to define theme:
- i18n variables: (default translated string)
        header.subjecti18n
        header.replyToi18n
        header.toi18n
        header.cci18n
        header.bcci18n
        header.fromi18n
        header.spamstatusi18n
        header.datei18n
        header.resentfromi18n
        header.vcardi18n
        header.resenttoi18n

- theme variables:
        header.absoluteThemePath
        header.subject
        header.replyTo
        header.replyToStr
        header.to
        header.toStr
        header.cc
        header.ccStr
        header.bcc
        header.bccStr
        header.from
        header.fromStr
        header.spamHTML
        header.dateshort
        header.date
        header.useragent
        header.xmailer
        header.resentfrom
        header.organization
        header.vcardname
        header.activecolordark
        header.fontcolor
        header.linkcolor
        header.photowidth
        header.photoheight
        header.applicationDir
        header.subjectDir
        header.photourl
        header.isprinting
        header.resentto