Custom emoji preview against both dark and light backgrounds

Currently, the dialog for adding custom stickers to your community gives you a preview of what the sticker looks like against a dark and a light background. This is great for testing accessibility. The same doesn't happen for adding custom emoji, though. Testing for sufficient contrast against both types of backgrounds is arguably even more important here than for stickers, given that emoji are smaller and custom emoji are likely to be more frequent than custom stickers.

A dark- and light-background preview is added to the dialog for adding custom emoji. (Maybe also other previews, such as reaction pills, since those have yet another background color if you're among the people who reacted.)