Notification-Emails in HTML-Format include two references to CSS_stylesheets with the following URLs:
http://<server>/<context>/s/614/9/1.0/_/download/resources/jira.webresources:global-static/global.css
http://<server>/<context>/s/614/9/1.0/_/download/resources/jira.webresources:global-static/global-static.css

While the former works, the latter yields Apache-Error 404, file not found.

Inspecting atlassian-jira\WEB-INF\classes\system-webresources-plugin.xml I found the working resource
<resource type="download" name="global.css" location="/styles/global.css" >
in web-resource <web-resource key="global-static" ...>, but did not find the other.

(Maybe this is correct and it should deliver a combined CSS from all the resources under web-resource global-static?)

The source of the problem seems to be the velocity-file in
atlassian-jira/WEB-INF/classes/templates/email/html/includes/header.vm.