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You have the option to create a custom Trigger and then embed the issue collector in a external HTML with your custom CSS.
there is a brand new App called "ReVitalized Issue Collector for Jira" in Atlassian Marketplace, you can style the Form with your own CSS and it even adds a Google reCAPTCHA feature for better security and anti spam.
It is reusing the existing Jira Issue Collector.
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