
Hi Fabian,
If you have a Pro license, you can download the full source of all our templates after logging into your organization's account on our website.
For that specific Border, there is not an easy way to target it with a style since it is unnamed. The Background is based on the UserPromptControl.TrayBackground property, BorderBrush is based on the UserPromptControl.BorderBrush property, BorderThickness is fixed at 1px for the top border.
The ControlTheme applied to UserPromptControl is implicit based on the control type so you'd just need to base any custom theme on {StaticResource {x:Type actipro:UserPromptControl}}
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As a word of caution, the next release of our Avalonia controls (releasing very soon) is updating how the status/footer icons are defined. There are some properties being renamed and the template for displaying the icons is changing. If you customize the template, make sure you look for changes in the next release and apply those to your template as well.
Would you mind sharing what you are hoping to change? Depending on how widely applicable the change is, we might be able to accomodate the change for the next release.