Highlighting lexers are part of the [Scintilla](https://www.scintilla.org/)
project, if an existing lexer isn't good enough then a new one must be added
there first. You can look at using a different (existing) lexer in a [custom
filetype](https://www.geany.org/manual/current/index.html#custom-
currently only "vanilla" html parts of templates would have highlighting - the
rest is recognized as plain text. And adding .jinja/.j2 extension to file
without specifying in header (say, if file extends other
templates) will break highlighting entirely
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