Add ability to change themes in the previewer.

[BobbyCannon]

Is your feature request related to a problem? Please describe.

No

Describe the solution you'd like

Would like the ability to view the preview in the theme colors.

Would like to see the preview (yellow arrow) in the dark (or other) theme (green arrow).

Describe alternatives you've considered

No response

Additional context

I have tried to contribute but I have failed. I will continue to try.

[maxkatz6]

It probably should use IDE theme by default (if it's possible to retrieve), and it also should remember selected option if custom.
I am not familiar with VS plugin API side, but let me know if you need help with Avalonia side.