ContentDialog Border#BackgroundElement has hardcoded margin

[spacilv]

Hi, is there any reason why Border#BackgroundElement within ContentDialog template has harcoded margin (v2.3.0)? We would like to change the margin by custom style but our value is overriden from the template. The margin was not hardcoded in the v2.0.5. Thank you.

[viktorspacil]

Issue #664 was created with pull request proposal.