Summary of the pull request

Two bugs are fixed:

• Due to a bug in AdaptiveCards, Positive and Destructive buttons are rendered with incorrect styling. It seems to be Windows 10 styling with square corners. Override the styling of Positive buttons to be Xaml's accent buttons, and make destructive buttons normal for now. We may choose an accessible foreground color in the future to match default behavior, which is to make the foreground color the "attention" color from the AC's HostConfig
• Choose an accessible color for "attention" text as it is used today.

References and relevant issues

48295223, 48295257

Detailed description of the pull request / Additional comments

Validation steps performed

PR checklist

• Closes Accessibility fixes for 0.12 milestone #2324
• Closes Possible contrast issue with active and disabled "Positive" widget buttons #1964
• Tests added/passed
• Documentation updated