There shouldn't be any issues using Danish in the template emails. You should be able to do it just by following the example templates included in the FBA Pack. I'd suggest changing the templates a little at a time, and testing them as you make the
changes - so that you can pinpoint the change in the template that is causing issues.
As for the style sheet - you should be able to just add <style> tags to the email body. I'd get a standard email formatted properly in your regular email program first, and then work on moving it to the template, a small piece at a time - to work out
any issues as you go. For adding a logo, you may just want to add an <img> tag with the src pointing to a logo image that's hosted online.
Also, make sure that you're using version 1.3.1 of the FBA Pack, as there were some improvements to sending the template emails in version 1.3.0. If you have version 1.3.0 or greater, you can check the version from the Site Settings -> FBA Site Configuration